PURPOSE: To control the number of revolutions of a motor stably and reliably regardless of leading phase or delayed phase of a current flowing through a single-phase induction motor.

CONSTITUTION: A triac 13 for controlling a phase angle of an applied voltage to a single-phase induction motor 12, a controller 15 for sweep-oscillate firing pulses of 0 to 180 degrees with a power supply voltage zero cross point as starting point during the initial power supply closing for the motor 12, Hall IC14 for detecting the number of revolutions of the motor 12 corresponding to each firing angle of the above, and a nonvolatile memory 18 for memorizing the number of revolutions of the motor in response to each firing angle of the firing pulses are provided. The firing pulses of a required firing angle corresponding to the number of revolutions of the motor are given to the triac 13 by the controller 15, and the number of revolutions of the motor is controlled to the number of revolutions corresponding to this firing angle.