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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 US2r1HfSrManh+azoeae7n8mUehhc9YvHNVfK3sxq3BLLnC32WDv6Xu8InHaRX1N
 SMx1YIuhnTjWRsuEVESevA==

<SEC-DOCUMENT>0001257584-10-000006.txt : 20100302
<SEC-HEADER>0001257584-10-000006.hdr.sgml : 20100302
<ACCEPTANCE-DATETIME>20100302183320
ACCESSION NUMBER:		0001257584-10-000006
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20100227
FILED AS OF DATE:		20100302
DATE AS OF CHANGE:		20100302

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Radecki Brian J
		CENTRAL INDEX KEY:			0001402193

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-24531
		FILM NUMBER:		10650824

	MAIL ADDRESS:	
		STREET 1:		C/O COSTAR GROUP, INC.
		STREET 2:		2 BETHESDA METRO CENTER, 10TH FLOOR
		CITY:			BETHESDA
		STATE:			MD
		ZIP:			20814

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			COSTAR GROUP INC
		CENTRAL INDEX KEY:			0001057352
		STANDARD INDUSTRIAL CLASSIFICATION:	SERVICES-BUSINESS SERVICES, NEC [7389]
		IRS NUMBER:				522091509
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		2 BETHESDA METRO CENTER
		STREET 2:		10TH FLOOR
		CITY:			BETHESDA
		STATE:			MD
		ZIP:			20814
		BUSINESS PHONE:		3012158300

	MAIL ADDRESS:	
		STREET 1:		2 BETHESDA METRO CENTER
		STREET 2:		10TH FLOOR
		CITY:			BETHESDA
		STATE:			MD
		ZIP:			20814
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-02-27</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001402193</rptOwnerCik>
            <rptOwnerName>Radecki Brian J</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>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Financial Officer</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-02-27</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>555</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>39.35</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>25298</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 6,094 shares of Common Stock and 19,204 shares of Common Stock subject to unvested Restricted Stock Grants.  The shares of Common Stock owned and set out in this line item include a total of 809 shares purchased to date by the reporting person pursuant to bi-weekly purchases made under the Company's Employee Stock Purchase Plan.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Jonathan Coleman, Attorney-in-Fact</signatureName>
        <signatureDate>2010-03-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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