The present invention has an object to provide an inverter system for a vehicle-mounted air conditioner that prevents noise from entering a control circuit and allows high speed communication. A motor control microcomputer 24 is isolated from a gate circuit 22, and communication therebetween is performed via a photocoupler 80. This suppresses the influence of noise resulting from changes caused by changes in voltage and current that occur in a high voltage system for driving a motor 30. Also, the motor control microcomputer 24 and a communication driver 27 can be directly bus-connected without a photocoupler, thereby allowing high speed communication therebetween.