The present invention seeks to realize an optical device that is capable of preventing stray light from entering light input and output means such as optical fibers and that excels in return loss characteristics. The optical device has a free space at least having a wall surface at part thereof, one or more light output means that outputs a light beam toward the free space and one or more light input means that inputs the light beam arriving through the free space. The optical device further includes an antireflective means such as a terminal waveguide provided at either part of the wall surface to prevent unwanted light irradiated to that part of the wall surface from being reflected to the free space.