The optical sensor includes a photodiode PD1 with its anode grounded, a diode group DG1 having one end connected to a cathode of the photodiode PD1, a current source I1 having one end connected to the other end of the diode group DG1, a power supply section for applying a constant voltage to the other end of the current source I1, and a grounded-emitter NPN transistor QOUT1 having a base connected to the cathode of the photodiode PD1 and a collector connected to one end of the current source I1. The diode group DG1 is provided by n diodes D1, D2, . . . , Dn connected in series so that their forward directions are directed toward the photodiode PD1 side, and a photocurrent which flows from a connecting point between the diode group and the current source to the photodiode is converted into a voltage and outputted as a photoelectric conversion signal.