PURPOSE: To improve the accuracy of an optical A/D converter, by varying the scanning cycle according to the output of a 1-dimensional photodetecting array to which the divided beams are supplied.

CONSTITUTION: The divided beams are supplied to a CCD which functions as a 1-dimentional photodetecting array. Then a common power supply 11 of the CCD is connected to a positive power supply, and a common output terminal 12 is connected to an arithmetic amplifier 15. The output of the amplifier 15 is averaged by a smoothing circuit 17 to obtain the voltage corresponding to the mean value of the light volume irradiated to each phototransistor 8. This voltage is supplied to a voltage control oscillator 18 to obtain a pulse signal of a frequency corresponding to the voltage. This signal is used as a scan signal to be supplied to a control signal terminal 14 of a shift register. In such way, the scanning cycle is controlled according to the output of the CCD to increase the accuracy of an optical A/D converter.