To provide an optical wavelength which enable the measuring of the wavelengths of a plurality of measuring light.

Reference light 18 and a plurality of measuring light 20 are branched into reflected light 18a and 20a and transmitted light 18b and 20b by a beam splitter 2. The reflected light 18a and 20a are reflected by a fixed mirror 3, the transmitted light 18b and 20b are reflected by a moving mirror 4, and then arranged into the beam splitter 2. The reference interference fringes 19a and measurement interference fringes 22a corresponding to the movement of the movable mirror 4 are converted from light to electric signals by a light receiving devices 6 and 7, and the number of waveshapes of the obtained reference fringe signals 19b and 22b are counted by counting parts 8 and 9. With this number of the waveshapes as a storage address, the counted values of a counting part 11 which counts a high speed rectangular waves 23 for the period from the start to stop of the movement of the moving mirror 4 each are stored in a reference light RAM part 12 and measuring light RAM part 13. Then the wavelength of the measuring light 20 is computed from the values of the addresses of the reference light RAM part 12 and measuring light RAM part 13, the data, and the wavelength of the reference light 18.