There are provided a photodiode 11, an inverting amplifier 12 which includes an operational amplifier 12a and a first resistor 12b, a second resistor 21 which has a smaller resistance value than the first resistor 12b and which is inserted between the positive side of a power supply and the photodiode 11, a differential amplifier 22 which outputs a voltage across both ends of the second resistor 21, and a diode 23 which is connected between a ground and a connection point between the photodiode 11 and an inverting input terminal of the operational amplifier 12a and which changes to a non-conductive state when the inverting amplifier 12 is not saturated and changes to a conductive state when the inverting amplifier 12 is saturated so that the output current of the photodiode 11 is bypassed.