A transmission ratio of a rotational angle of an output shaft to a rotational angle of an input shaft is adjustable by a transmission ratio changing mechanism when a coupling member is in a unlocked position. A resilient member urges the coupling member toward the locked position. A solenoid holds the coupling member at the unlocked position against a resilient force of the resilient member. A solenoid drive control apparatus applies a drive voltage to the solenoid to hold the coupling member positioned at the unlocked position and reduces the drive voltage through a voltage attenuation process before the coupling member finally reaches the locked position. During the voltage attenuation process, the solenoid produces an electromagnetic force yielding to the resilient force of the resilient member and decreases a shifting speed of the coupling member on the way to the locked position.