<SUBMISSION>
<ACCESSION-NUMBER>0001057352-08-000025
<TYPE>4
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20080417
<FILING-DATE>20080421
<DATE-OF-FILING-DATE-CHANGE>20080421
<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>08768070
</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>2008-04-17</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</value>
            </securityTitle>
            <transactionDate>
                <value>2008-04-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>720</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>45.39</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>206775</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents all shares of Common Stock owned, consisting of 127,976 shares of Common Stock and 78,799 shares of Common Stock subject to unvested Restricted Stock Grants.  The reporting person was issued 39 shares of Common Stock on September 10, 2007, as a service award from the issuer on his ten-year anniversary of employment, on which date the closing stock price of the Common Stock was $51.57.  The 39 shares were inadvertantly omitted from previous filings, but are included in the current total of shares directly held.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Jonathan Coleman, Attorney-in-Fact</signatureName>
        <signatureDate>2008-04-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
