PURPOSE: To shorten measuring steps by measuring the power of the light beam from an actuator, and measuring the increase and decrease of the optical power which a are associated with the operation as the speed and the stroke of the actuator.

CONSTITUTION: The actuator of material to be measured 5 is connected to an operation amplifier 3. Thus the actuator is driven. A light pickup is further connected to a light-output adjusting amplifier 4, and the light is emitted. The light beam is detected with a photodetector 1. Under this state, the optical power is increased and decreased with the movement of the actuator. Namely, when the light pickup approaches the photodetector 1, the amount of the light increases. When the light pickup goes away, the amount of the light is decreased. Therefore, the increasing and decreasing speeds of the light power are corrected with an analyzer, and the speed of the actuator is obtained. The absolute values of the increase and the decrease are corrected with the analyzer, and the values are made to be the strokes of the actuator. The output of the photodetector 1 at the maximum level is measured as the amount of the light power.