To provide an optical heterodyne detection system capable of generating a heterodyne signal having an improved S/N ratio.

This optical system for heterodyne detection is equipped with an optical combining means 210 for generating a combined optical signal by combining an input signal 202 with a sweep local oscillation signal 206, and outputting beams including respectively a part of the combined optical signal, and a pass band optically arranged so as to receive the beams outputted by the optical combining means 210, for tracking the wavelength of the sweep local oscillation signal 206. The system includes an optical pre-selector 214 for outputting a filtered parts of the beams, and photo detectors 212 optically arranged respectively so as to receive different parts of the filtered parts of the beams, for generating electric signals in response to each filtered part of the beams.