Light under measurement whose wavelength is continuously swept is incident on fiber-optic Etalon. The fiber-optic Etalon transmits the light under measurement each time the wavelength of the light under measurement satisfies specific conditions. A PD detects the transmitted light of the fiber-optic Etalon and outputs the intensity of the light under measurement. A counter counts the number of peaks of the output of the PD. A CPU calculates the wavelength of the light under measurement based on the count value of the counter.