An optical switch changes over transmission paths of an optical signal by
a refractive index change. The optical switch has an optical waveguide
layer having an optical waveguide whose output path of an optical signal
branches into two, and reflection diffusing ditches which are provided in
a portion outside the optical waveguide in the optical waveguide layer,
wherein the portion is closed to a branch portion of the optical
waveguides. The reflection diffusing ditches extend in a direction
crossing a traveling direction of the optical signal.