<SUBMISSION>
<ACCESSION-NUMBER>0001057352-07-000078
<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>Reynolds Catherine B
<CIK>0001292209
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4
<ACT>34
<FILE-NUMBER>000-24531
<FILM-NUMBER>071122656
</FILING-VALUES>
<BUSINESS-ADDRESS>
<PHONE>703-442-3086
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>PO BOX 9870
<CITY>MCLEAN
<STATE>VA
<ZIP>22102
</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>0001292209</rptOwnerCik>
            <rptOwnerName>Reynolds Catherine B</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>CATHERINE B. REYNOLDS FOUNDATION</rptOwnerStreet1>
            <rptOwnerStreet2>1676 INTERNATIONAL DRIVE, SUITE 501</rptOwnerStreet2>
            <rptOwnerCity>MCLEAN</rptOwnerCity>
            <rptOwnerState>VA</rptOwnerState>
            <rptOwnerZipCode>22102</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>1650</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5736</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 1,021 shares of Common Stock and 4,715 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>
