PURPOSE: To prevent the overcurrent from flowing at the time when an excessive optical signal is inputted to a photoelectric transducing element by varying the magnitude of the optical signal inputted to the photoelectric converting element, and keeping a multiplication factor of the photoelectric transducing element constant.

CONSTITUTION: The optical signal of a path (a) for applying electric power to a Mach-Zehnder type modulator 41 transmits through as it is, but the refractive index of light is varied in a path (b). Therefore, by synthesizing the optical signal of the path (a) which passes through as it is and the optical signal which passes through the path (b) whose refractive index is different, the magnitude of the optical signal is varied. Accordingly, by adjusting electric power applied to the Mach-Zehnder type modulator 41, the magnitude of the optical signal is varied, and by keeping a bias current allowed to flow to an APD 11 constant, the band width of the APD 11 can be kept constant.