PURPOSE: To obtain a light receiving amplifier whose gain is switched at a high speed.

CONSTITUTION: A cathode of a photodiode PD is connected to a power source VCC via a current mirror circuit 1. An anode of the photodiode PD is connected to an input of an amplifier circuit AMP. The amplifier circuit AMP receives a light receiving current of the photodiode PD and converts the light receiving current into a voltage and amplifies to output an output signal VOUT. A current control signal equal to the light receiving current outputted from the current mirror circuit 1 is given to a control input terminal of a gain changeover circuit 2. The gain changeover circuit 2 consists of a resistor Rf1, a resistor Rf2 and a switch SW and the resistor Rf1 is connected between the input and output of the amplifier circuit AMP and the resistor Rf2 and the switch SW connected in series are connected between the input and output of the amplifier circuit AMP.