A motor driver includes an inverter, a multiphase pre-driver circuit, a controller and motor relays. The inverter includes pairs of an upper-arm switching element and a lower-arm switching element being connected in series between a ground line and a power supply line, and supplies power to a multiphase motor by converting direct current power of a battery. The multiphase pre-driver circuit drives the upper-arm and lower-arm switching elements. The controller commands the multiphase pre-driver circuit to drive the switching elements, and controls electrical conduction from the inverter to the multiphase motor. The motor relays interrupt a current flowing from the multiphase motor to the inverter during an off state. The multiphase pre-driver circuit includes a charge pump. The motor relays are turned on by an output voltage of the charge pump during an operation of the charge pump in a situation apart from that the controller provides an instruction.