An optical difference detector includes a first APD and a second APD, a first voltage application unit that applies a first bias voltage to the first APD and a second voltage application unit that applies a second bias voltage to the second APD, a differential amplifier that is connected in parallel to the first APD and the second APD and amplifies a difference between a first signal current output from the first APD and a second signal current output from the second APD, and a feedback control unit that controls the second bias voltage so that a low frequency component of a first monitoring current in the first APD and a low frequency component of a second monitoring current in the second APD are equal.