PURPOSE: To obtain a practicable means for identifying optical path difference which withstands use on a site where environmental conditions are unfavorable in terms of reliability, profitability oscillation resistance, etc. by enabling the exact identification of the optical path difference between an optical path by a stationary mirror and an optical path by a movable mirror form a driving current value.

CONSTITUTION: DC current is conducted to a voice coil 1 and the moving mirror M' connecting to the coil 1 is driven by the mutual effect of a magnetic field generated by said coil and a magnet 2 and for the characteristic indicating the relation of the moving distance of the mirror M' with the direct current, it is easy to obtain the characteristic of good reproducibility if the time is short. More specifically, if the time required for moving the mirror M' for one forward and backward cycle is less than several seconds, the temp. in an interfering part and the magnetic characteristic of the magnet 2 are maintained constant during said time. If the relation between the driving input (DC power source) and moving distance of the mirror M' is preliminarily determined at one or 2 places during the moving stroke of the mirror M exactly by certain means, namely, if it is identified preliminarily, the moving distance with respect to the other driving input value is determined by an interpolation or extrapolation method.