Provided is a control device for an electric motor drive unit that is capable of altering system voltage while suppressing the vibrations of an AC motor even during the square wave control, and that is capable of expanding the region used in square wave control, which has minimal switching loss, into the minimum rotation speed range. The control device is equipped with a switching control unit that controls the on-off state of a plurality of switching elements equipped with DC-AC conversion units and performs square wave control, in which square wave voltages having a plurality of phases are output. In cases where the system voltage (Vdc) changes during square wave control, the switching control unit performs square wave range adjusting control that sets the on-off timing of the plurality of switching elements on the basis of the rate of change (K) of the system voltage (Vdc) in a manner such that the time integral of the square wave voltage in each phase within the control cycle (T), which is set at the length of the integral multiple of an electrical angle, is roughly identical in each phase.