In an optical detection method that requires a reference light such as homodyne detection, a signal light and the reference light must be equal to each other in the wavelength, and the phase relation between them must be maintained constant. In order to satisfy this requirement, the signal light and the reference light are extracted from the same light source and made equal to each other in the wavelength. The signal light and the reference light are transmitted so as to be temporally superimposed on each other with orthogonal polarizations to the same optical path, thereby making the external environments equal to each other to maintain the constant phase relation.