<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-05-27</periodOfReport>
    <issuer>
        <issuerCik>0000874977</issuerCik>
        <issuerName>STATE AUTO FINANCIAL CORP</issuerName>
        <issuerTradingSymbol>STFC</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001184083</rptOwnerCik>
            <rptOwnerName>LOWTHER JOHN R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>Senior Vice President</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeSecurity><securityTitle><value>Common Shares without par value</value></securityTitle><transactionDate><value>2003-05-22</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F1"/></transactionCoding><transactionAmounts><transactionShares><value>0</value></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>51320.774</value><footnoteId id="F2"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><derivativeSecurity><securityTitle><value>Employee Stock Option (Right to Buy) NQ</value></securityTitle><conversionOrExercisePrice><value>18.74</value></conversionOrExercisePrice><transactionDate><value>2003-05-22</value></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>14500</value></transactionShares><transactionValue><value>0</value><footnoteId id="F1"/></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>2004-05-22</value><footnoteId id="F3"/></exerciseDate><expirationDate><value>2013-05-21</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common shares</value></underlyingSecurityTitle><underlyingSecurityShares><value>14500</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>14500</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></derivativeSecurity><footnotes><footnote id="F1">field should be left blank, but due to a software defect, requires input.</footnote><footnote id="F2">Includes indirect holdings  of 11,984.000 shares.  Also includes the following acquisitions:  22.498 shares acquired in March 2003 under the dividend reinvestment feature of the STFC Employee Stock Purchase Plan, and 0.474 shares acquired in March 2003 under the STFC Dividend Reinvestment Plan.</footnote><footnote id="F3">The options vest in three equal annual installments beginning on May 22, 2004.</footnote></footnotes><ownerSignature><signatureName>John R. Lowther</signatureName><signatureDate>2003-05-27</signatureDate></ownerSignature></ownershipDocument>

