An insulation failure diagnosis apparatus is provided which diagnoses presence/absence of insulation failure between a body of a vehicle and a connection path to a power supply for a traction unit. The apparatus includes a power converter circuit connected to the power supply and supplying power to an in-vehicle auxiliary unit, an outputting unit outputting an AC signal to the connection path, and a diagnosis unit diagnosing presence/absence of the insulation failure based on a current flowing through the connection path when the AC signal is outputted. The diagnosis unit includes a storage unit storing a determination value for determining the insulation failure or information for setting the determination value. The determination value or the information is obtained by measuring an electrical state quantity of the connection path depending on stray capacitance and is dielectric resistance of the connection path obtained when an AC signal is outputted to the connection path.