PURPOSE: To automatically remove the offset voltage of a current controlling circuit, by detecting a digital command signal value equal to the offset voltage value of the current controlling circuit and storing it in a state that current is not conducted to a driving motor after a power source is made.

CONSTITUTION: Immediately after a power source is made, a change-over switch 6 is switched to the side of feed zero volt potential. After that, the value of digital command signal B is reduced gradually from a specified value. The signal is subjected to a D/A-conversion, and the output is generated as the current command signal of an analog. In this case, current is not conducted to a driving motor 8, and so an only offset voltage value is fed back. Then, the digital command signal value B when the level of the output signal of a comparator 4 is transferred, is stored. Besides, the stored digital command signal value is set to be of a zero level, and following digital command signal is set.