Provided is a control device for an electric motor which includes multiple energization systems each including an inverter and coils corresponding to different phases, and which generates a steering assist force in an electric power steering system. Control device diagnoses whether any of inverters 1A and 1B fails, and, when either of the inverters is diagnosed as having failed, reduces the output ratio of the failing inverter to 0% while increasing the output ratio of the normally operating inverter to 100% so as to prevent a drop in the total output from all the inverters after the failure-positive diagnosis. If steering operation is performed after the failure-positive diagnosis, control device performs overheat protection processing for gradually lowering the limit value for the total output from all the inverters in accordance with the sensed temperature of the normally operating inverter.