A feline vaccinated with an FIV vaccine produces an antibody against an FIV. Therefore,in a serodiagnosis using an FIV and an FIV-derived substance, it is difficultto distinguish between a feline who has been vaccinated with an FIV vaccine anda feline who is infected by an FIV. It becomes now possible to determine the presenceor absence of the history of FIV vaccination in a feline by detecting an antibodythat is produced by the vaccination with an FIV vaccine but is not produced by theinfection by an FIV in the feline. It is possible to easily determine whetheran anti-FIV antibody-positive feline is infected by an FIV or has been vaccinatedwith an FIV vaccine.