PURPOSE: To effect monitoring of a laser light output modulated at a high speed irrespective of polarity of a photodiode.

CONSTITUTION: A photodiode 2 receives part of an optical output of a semiconductor laser 1, and outputs a current responsive to a received power. The photodiode 2 is connected at its cathode side to the laser 1. A differential amplifier 3 inputs a negative reference voltage -VR and an output current im of the photodiode 2, and outputs a voltage responsive to the output current with the negative reference voltage as a reference. With the structure, the photodiode 2 can be reversely biased, and a responding speed of the photodiode 2 can be accelerated.