PROBLEM TO BE SOLVED: To reduce the size of a sensor by reducing the scale of a signal processing circuit by switching the input of a plurality of detected signals outputted from a light receiving lens to a signal processing means at such timing that the input is asynchronously time-divided with switches. SOLUTION: Light rays made incident on PDs (light receiving elements) 1 and 2 are transduced into electric currents and detected signals I1 and I2 are intermittently sent to a light receiving circuit 8 through switches SW1 and SW2. The switches SW1 and SW2 are alternately switched to each other at the clocks of oscillating pulse signal SG1 and SG2 supplied from an oscillation circuit 7 and the signals I1 and I2 passing through the switches SW1 and SW2 are sent to the circuit 8 at such timing that the signals are asynchronously time-divided. Since the detected signals are processed by signal processing means of one system in paired states, the same number of light receiving circuits 8, amplifier circuit 9, S/H circuits 10, A/D conversion circuit 11, etc., as that of light receiving outputs is not required and the scale of a signal processing circuit can be reduced.