To reduce the temperature change in, for example, the differential efficiency of the output of an optical fiber by enabling the reflection factor of a light-transmitting film on the surface of a light-receiving element to have a large wavelength dependence and maximizing the connection efficiency between a semiconductor element and an optical fiber at a temperature higher than the room temperature.

Output from the front of a laser diode(LD) 15 is connected to an optical fiber via a lens 2, and rear output is inputted to a light-receiving element (PD) for monitoring. Coating films 12-14 are forned on a surface 11 of the PD 16 as a light- transmitting film, thus achieving a wavelength dependence, where a reflection factor decreases as the wavelength increases. The oscillation wavelength of the LD 15 increases as the temperature increases, and the reflection factor of the light transmitting film decrease as the temperature increases, thus improving the transmission rate of the light-transmitting film as the temperature increases. A position is adjusted so that the connection efficiency between the LD 15 and an optical fiber 3 can be maximized at a temperature higher than the room temperature, and the differential efficiency of fiber output 6 and the temperature change in the current value for monitoring the PD 16 are mode decreased.