A failure diagnosis system that erases diagnostic information applied to a vehicle that includes an ECU for that stores a vehicle identification number (VIN) and diagnostic information (hereinafter, referred to as “VIN storage ECU”), and an ECU that does not store the vehicle identification number but stores the diagnostic information (hereinafter, referred to as “other ECU”) is characterized by being constructed so as to erase the diagnostic information stored in the VIN storage ECU and also erase the diagnostic information stored in the other ECU in connection with the writing of the vehicle identification number stored in the VIN storage ECU.