A reverse impulse is applied to the stepper motor (1) to rotate the stepper motor in the opposite direction when no detect signal is output from the fixed sensor (5) when the stepper motor stops after the output of a detect signal from the fixed sensor. A pause or stop position at which the stepper motor stops after rotation in the opposite direction is assessed as the regular null position when the fixed sensor again outputs a detect signal during the rotation of the motor in the opposite direction. A forward impulse is again applied when no detect signal is generated during reverse rotation.
