To provide a method for inspecting an electric heater that is included as an auxiliary heat source in a vehicle heating device and can constantly and accurately determine whether or not a wire connection state of a connector of the electric heater is normal.

Each of "Duty rate (Ra)" and "Duty rate (Rb)" of an alternator 53, before and after the electric heater 51a is operated, is measured. The wire connection state of the connector of a wire harness 51d is diagnosed by comparing "Duty rate change (Rc)" of the "Duty rate (Ra) and Duty rate (Rb)" with a predetermined "specified value (R)".