There is provided a controller for a steering device that makes it possible to prevent sudden ceasing of steering assisting power by effectively utilizing an auxiliary power source. An electric power steering device that generates steering assist force by a motor includes a battery for supplying electric power to the motor, an auxiliary power source that supplies the motor with electric power, and a control circuit for controlling a power supply to the motor. In the event of a breakdown of the battery, the control circuit adjusts a power supply to the motor from the auxiliary power source according to an amount of energy remaining in the auxiliary power source so as to reduce steering assist force.