An optical device enabling reducing device scale is constituted by M circulators (11a, 11b), (M being a natural number), a waveguide-type diffraction grating (12) including M first input/output waveguides (121a, 121b) formed at its one end and M.times.N second input/output waveguides (125a-1 to 125a-N and 125b-1 to 125b-N) formed at the other end, (N being a natural number), and paths from the M circulators being connected to the M first input/output waveguides; and N reflection-type optical switches (13-1 to 13-N) each for reflecting the optical paths for M adjacent waveguides out of the M.times.N second input/output waveguides (125a-1 to 125a-N and 125b-1 to 125b-N) and for switching over the returning-destination waveguide respectively, wherein the M circulators, the waveguide-type diffraction grating and N reflection-type optical switches are arranged in cascade. Consequently, the optical device can function preferably also as a wavelength selective optical switch available in a WDM transmission system.