<SUBMISSION>
<ACCESSION-NUMBER>0001057352-07-000069
<TYPE>4
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20070914
<FILING-DATE>20070918
<DATE-OF-FILING-DATE-CHANGE>20070918
<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>LOW JOSIAH O III
<CIK>0001016215
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4
<ACT>34
<FILE-NUMBER>000-24531
<FILM-NUMBER>071122538
</FILING-VALUES>
<BUSINESS-ADDRESS>
<STREET1>877 PARK AVENUE
<CITY>NEW YORK
<STATE>NY
<ZIP>10172
<PHONE>2128923000
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>277 PARK AVENUE
<CITY>NEW YORK
<STATE>NY
<ZIP>10172
</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>2007-09-14</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001016215</rptOwnerCik>
            <rptOwnerName>LOW JOSIAH O III</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>100 CEDAR CLIFF ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>RIVERSIDE</rptOwnerCity>
            <rptOwnerState>CT</rptOwnerState>
            <rptOwnerZipCode>06878</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2007-09-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1934</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12724</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Shares owned by Mr. Low's wife, for which he disclaims beneficial ownership</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents all shares of Common Stock owned, consisting of 7,197 shares of Common Stock and 5,527 shares of Common Stock subject to unvested Restricted Stock Grants.  The previous Form 4, filed by the Reporting Person on October 26, 2006, inadvertantly set forth 10,970 shares in column 5 of Table I rather than 10,790 shares, consisting of 6,551 shares Common Stock and 4,239 shares of Common Stock subject to unvested Restricted Stock Grants.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Jonathan Coleman, Attorney-in-Fact</signatureName>
        <signatureDate>2007-09-18</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
