To obtain high wavelength resolution over a wide wavelength range.

A first polarization conversion element 23 formed so that intensity ratio obtained from emission light to the monotonous change of a wavelength in a specified range changes monotonously, and a second polarization conversion element 24 formed so that the intensity ratio obtained from emission light to the monotonous change of a wavelength in a specified range changes periodically are used. A wavelength closest to the wavelength obtained from the emission light of the first polarization conversion element 24 is selected as the wavelength of light to be measured from a plurality of candidate wavelengths obtained with high wavelength resolution from the emission light of the second polarization conversion element 24 when the light to be measured enters.