PURPOSE: To obtain the detection circuit whose cost is reduced and whose mounting space is reduced by a method wherein light-emitting parts for a plurality of photosensors emit light sequentially and in a time-sharing manner.

CONSTITUTION: Photosensors are constituted of light-emitting elements 4-1 to 4-n inside a light-emitting part 4 and of photodetectors 5-1 to 5-n inside a light- receiving part 5 which is faced with it. A single amplification part 6 and a single comparison part 7 are installed for the photosensors of (n) pieces, and they are driven and controlled by an oscillator 1, a shift register 2 and a driver 3 in such a way that the light-emitting part 4 is turned on in a time-sharing manner. That is to say, the light-emitting elements 4-1 to 4-n in a plurality are turned on sequentially, signals in the light-receiving part 5 are picked up sequentially, and an output level is detected. The comparisonr part 7 compares a reference voltage value decided by resistances 71, 72 with an output signal from the amplification part 6. Lastly, a serial-data conversion part 8 outputs, in synchronization with the oscillator and serially, the output of the comparison part 7 as a detection result corresponding to every photosensor. In this manner, a plurarilty of photosensor signals are processed by a single detection circuit.