A gain switching circuit (3) that switches a conversion gain of a preamp (2) , which outputs a voltage signal upon amplifying an output current of a photo-detecting element (1) that converts a burst-form optical signal into an electrical signal and with which a series circuit, formed with a first resistor (6) and a first switching element (9) , and a series circuit, formed with a second resistor (7) and a second switching element (10) are connected in parallel to a feedback resistor (2B). The switching circuit further includes a gate signal generating circuit that generates, upon receiving an output of the pre-amplifier, a gate signal for a first and second operating unit.