PURPOSE: To achieve accurate optical switching function regardless of the change in ambient temperature by controlling the power supply to a thermal element so as to form a prescribed gradient of refractive index based on the temperature detected on both the upper and lower surfaces of optic material.

CONSTITUTION: Temperatures TU and TD on the upper and lower surfaces of the dielectric crystal 16 having the temperature optical effect are read based on the detected signal in the 1st and 2nd temperature detecting devices 13 and 14 and the difference ΔT in temperature between TU and TD, that is, ΔT=TU- TD is calculated. Next, the reference temperature difference ΔTn corresponding to the discriminated mode is read from the memory and compared with the temperature difference ΔT, and if those temperature differences are the same, the resistance value of variable resistance 23a is not adjusted. However, if difference ΔT is greater than difference ΔTn, the resistance value of resistance 23a is adjusted upward by a constant value, thereby reducing the power supplied to Peltier element 7 to make the angle of deflection of incident light beam (b) small. If difference ΔT is less than difference ΔTn, the resistance value of resistance 23a is made low by a constant value to enlarge the angle of deflection of beam (b).