A controller for a motor and a control method for a motor change an operating condition of each motor, considering difference in operating conditions, such as loss and temperature, thus making it possible to reduce a total loss in a plurality of motors. The controller includes a first motor operating condition calculator and a second motor operating condition calculator which calculate the estimated values of losses incurred when a first motor and a second motor are driven, and a DC voltage control unit which carries out, on a motor having a largest estimated value of loss, the processing for changing a supply voltage to change the voltage of DC power supplied to inverters by a DC/DC converter, thereby reducing the difference between a phase voltage, which is the resultant vector of the voltages across terminals of the motor, and a target voltage set on the basis of an output voltage of the DC/DC converter.