PURPOSE: To remove the loss of diffracted light and to improve control accuracy by detecting the quantity of reduction of light on the non-diffracted light side at the time of diffraction and controlling the distribution ratio of the diffracted light to the non-diffracted light in accordance with the detected quantity.

CONSTITUTION: A difference between an output signal obtained at the OFF of diffracted light of an optical sensor PD and an output signal obtained at the ON of the diffracted light is extracted by a coupling capacitor C1 and detected and smoothed by diodes D1, D2 and a capacitor C2. The voltage is applied to one side of voltage determining resistors R3, VR for a constant voltage circuit consisting of TRs Q1WQ5, a Zener diode ZD, a variable resistor VR and resistors R3WR7 and the output voltage of the constant voltage circuit controls a voltage to be applied to a modulating means 2. Consequently, the reduced quantity of light of the non-diffracted light side at the time of diffraction is detected and the distribution ratio of the diffracted light to the non-diffracted light is controlled in accordance with the detected quantity, so that the diffracted light is not lost and the control accuracy can be improved.