PURPOSE: To monitor a light emission amount of a laser chip accurately even if a diffusion angle of laser beam emitted to the rear of a laser chip changes by reflecting rear light emitted to a reverse direction of laser beam from a light emission means to a specified inside space and by detecting intensity of light in the inside space thereafter.

CONSTITUTION: A photodetector 4 which is a detection means is attached to an inside of an inside integrating sphere 3 which is an inside reflection means, detects light for a part of a laser chip 2 which is a light emission means converted random diffused light and detects its light amount. Therefore, even if a diffusion angle of laser light L2 emitted from a rear 2b of the laser chip 2 is changed, there is no possibility of ratio between a whole light amount thereof and a light amount of laser beam detected by the photo detector 4 changing. As a result, a light emission amount of the laser chip 2 can be accurately monitored. Therefore, reliability of control by an automatic light amount adjustment circuit can be greatly improved.