PURPOSE: To detect a minute signal of a photo sensor in the low noise and low distortion state, through the charge/discharge of a capacitor from each unit in different timing, by providing the capacitor between a plurality of units having a photo sensor and a switching element each, and an integration circuit.

CONSTITUTION: A plurality units 81 are arranged in an array, a photo sensor 3 and a resistive element 70 are connected in series with a switching element 1 of each unit 81, and each capacitor 41 is charged and discharged. The output of each unit 81 is outputted to an output line 5, and given to a capacitor 51, a switching element 2 and an inverting terminal of an OP Amp 61 of an integration circuit 60, and a reference power supply 64 is connected to a non-inverting terminal of the Amp 61. A voltage on the output line 5 from each unit 81 is outputted in different timing according to the operation of the switch 1, the capacitor 51 is charged/discharge in the different timing and a minute signal from the photo sensor 3 is detected with a simple constitution.