The present invention has an object to provide an on-vehicle air conditioner that can prevent breakage or the like caused by applying a high voltage. In an inverter system 20 that controls an operation of a motor 30 of a compressor for the on-vehicle air conditioner, a motor control microcomputer 24 operates by converting a voltage supplied from an on-vehicle battery power source 50 into a low voltage. Thus, the motor control microcomputer 24 is operated to perform failure diagnosis of a high voltage circuit Cb without a high voltage power source 40 being turned on. The failure diagnosis can be performed without power being supplied from the high voltage power source 40 to the high voltage circuit Cb, thereby preventing the high voltage circuit Cb from being broken by applying the high voltage even when there is some failure in the high voltage circuit Cb.