<SUBMISSION>
<ACCESSION-NUMBER>0001057352-07-000008
<TYPE>4
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20070228
<FILING-DATE>20070302
<DATE-OF-FILING-DATE-CHANGE>20070302
<ISSUER>
<COMPANY-DATA>
<CONFORMED-NAME>COSTAR GROUP INC
<CIK>0001057352
<ASSIGNED-SIC>7389
<IRS-NUMBER>522091509
<FISCAL-YEAR-END>1231
</COMPANY-DATA>
<BUSINESS-ADDRESS>
<STREET1>2 BETHESDA METRO CENTER
<STREET2>10TH FLOOR
<CITY>BETHESDA
<STATE>MD
<ZIP>20814
<PHONE>3012158300
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>2 BETHESDA METRO CENTER
<STREET2>10TH FLOOR
<CITY>BETHESDA
<STATE>MD
<ZIP>20814
</MAIL-ADDRESS>
</ISSUER>
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>FLORANCE ANDREW C
<CIK>0001066061
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4
<ACT>34
<FILE-NUMBER>000-24531
<FILM-NUMBER>07668031
</FILING-VALUES>
<BUSINESS-ADDRESS>
<STREET1>C/O COSTAR GROUP INC
<STREET2>7475 WISCONSIN AVE., STE. 600
<CITY>BETHESDA
<STATE>MD
<ZIP>20814
<PHONE>3012158300
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>C/O COSTAR GROUP INC
<STREET2>7475 WISCONSIN AVENUE #600
<CITY>BETHESDA
<STATE>MD
<ZIP>20814
</MAIL-ADDRESS>
</REPORTING-OWNER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2007-02-28</periodOfReport>

    <issuer>
        <issuerCik>0001057352</issuerCik>
        <issuerName>COSTAR GROUP INC</issuerName>
        <issuerTradingSymbol>CSGP</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001066061</rptOwnerCik>
            <rptOwnerName>FLORANCE ANDREW C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O COSTAR GROUP, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>2 BETHESDA METRO CENTER, 10TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>BETHESDA</rptOwnerCity>
            <rptOwnerState>MD</rptOwnerState>
            <rptOwnerZipCode>20814-5388</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President and CEO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2007-02-28</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>31667</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>9.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>242797</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2007-02-28</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>31667</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>48.09</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>211130</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Option to Acquire Common Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>9.00</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2007-02-28</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>31667</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2008-06-30</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>31667</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Represents all shares of Common Stock owned, consisting of 174,832 shares of Common Stock and 67,965 shares of Common Stock subject to unvested Restricted Stock Grants.</footnote>
        <footnote id="F2">Average based on sale prices ranging from $ 48.04 to $48.27.</footnote>
        <footnote id="F3">Represents all shares of Common Stock owned, consisting of 143,165 shares of Common Stock and 67,965 shares of Common Stock subject to unvested Restricted Stock Grants.</footnote>
        <footnote id="F4">These options were fully vested as of 12/31/2000.</footnote>
        <footnote id="F5">Past reports filed by the reporting person showed aggregate option holdings in column 9, as opposed to holdings of a single class.  As of the date hereof, after the reported transactions, the reporting person owns options of various classes with the right to acquire in the aggregate up to 391,777 shares of Common Stock of the Company.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Jonathan Coleman, Attorney-in-Fact</signatureName>
        <signatureDate>2007-03-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
