An optical device including (a) a substrate having an electro-optic effect; (b) a modulating optical waveguide formed on a surface layer portion of said substrate and forming an interference optical modulator for modulating input light; (c) an output optical waveguide formed on said surface layer portion of said substrate and connected to a downstream side portion of said modulating optical waveguide; and (d) a branching monitoring section for monitoring branched light of light propagated along said output optical waveguide and emitted from an outgoing end face of said substrate. The output waveguide has a reduced width region in which the waveguide width is reduced.