<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-12-18</periodOfReport>
    <issuer>
        <issuerCik>0000874977</issuerCik>
        <issuerName>STATE AUTO FINANCIAL CORP</issuerName>
        <issuerTradingSymbol>STFC</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001184040</rptOwnerCik>
            <rptOwnerName>JOHNSON NOREEN W</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>518 E. BROAD STREET</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>COLUMBUS</rptOwnerCity>
            <rptOwnerState>OH</rptOwnerState>
            <rptOwnerZipCode>43215</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>Vice President</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeTransaction><securityTitle><value>Common Shares without par value</value></securityTitle><transactionDate><value>2003-12-18</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>G</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>5</value></transactionShares><transactionPricePerShare><value>0</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>9549.612</value><footnoteId id="F1"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeTransaction></nonDerivativeTable><footnotes><footnote id="F1">Includes the following acquisitions:  431.477 shares acquired in May 2003 and 137.508 shares acquired in November 2003 through the State Auto Financial Corporation Employee Stock Purchase Plan; 13.937 shares acquired in June 2003 and 14.127 shares acquired in September 2003 under the dividend reinvestment feature of the STFC Employee Stock Purchase Plan, and 0.244 shares acquired in June 2003 and 0.247 shares acquired in September 2003 under the STFC Dividend Reinvestment Plan.</footnote></footnotes><remarks/><ownerSignature><signatureName>Noreen W. Johnson by John R. Lowther, attorney in fact pursuant to POA filed with Commission 3-12-98</signatureName><signatureDate>2003-12-18</signatureDate></ownerSignature></ownershipDocument>

