PURPOSE: To accurately and stably control the amplitude of a vibration optical element and to easily perform the change setting of the amplitude by controlling an amplification factor variably based on a compared result with a decision value after measuring and integrating a time interval in accordance with the amplitude when a ray of light from a light source vibrates by the vibration optical element.

CONSTITUTION: A control part 15 is comprised of an integration circuit 16 which calculates the integrating value of the measuring value of a time interval and outputs an integration signal, memory 17 which stores a decision value in accordance with the reference time of the amplitude of spot light and outputs a decision value signal, and a decision circuit 18 which discriminates the magnitude of both the signals by receiving the integration signal and the decision value signal and outputs an amplification factor control signal to an amplifier circuit 2 based on a discriminated result. When the ray of light from a light source 4 vibrates by a resonant mirror 5, the time interval in accordance with the amplitude is measured. The amplification factor is controlled variably based on the discriminated result compared with the decision value set in advance after the time interval is integrated, and the amplitude of vibration of the resonant mirror 5 i.e., the scanning range of the ray of light is controlled.