An assist control portion, upon detecting source power supply abnormality of a main electric power source, switches the characteristic of an assist map in which the steering torque and the target current are related to each other from an original assist characteristic to an abnormality-time assist characteristic. Therefore, the steering handle operation suddenly becomes heavy, thereby causing the driver to be aware of the abnormality. After that, the abnormality-time assist characteristic is altered so that the assist force relative to the steering torque further decline in accordance with declines in the actual amount of charge of the subsidiary electric power source declines. Therefore, the steering handle operation gradually becomes heavier, thereby causing the driver to anticipate that the abnormality is further progressing.