To provide controller and control method for an electric motor including plural energization systems composed of an inverter and coils corresponding to plural phases. According to the present invention, if any abnormality is detected in one energization system being energized through first diagnosis processing, second diagnosis processing is executed on the energization system having the abnormality detected, which is switched to an unenergized state. Then, if the second diagnosis processing reveals occurrence of a short-circuit, a control gain of a normal energization system is lowered. In addition, a threshold used in the first diagnosis processing on the normal energization system is changed so as to make it difficult to detect an abnormality, hereby keeping the energization control on the normal energization system. Hence, if a short-circuit occurs in at least one of the energization systems, it is possible to avoid stopping the output from the normal energization system.