An optical device including (a) a substrate having an electro-optic effect; (b) an optical waveguide formed on a surface layer portion of said substrate and including an optical waveguide for performing optical modulation for light inputted to said substrate and an output optical waveguide and a monitoring optical waveguide branched from and connected to a downstream side portion of said modulating optical waveguide, said monitoring optical waveguide guiding light for monitoring optical modulation operation of said modulating optical waveguide; and (c) a reflecting portion being provided on the downstream side of said monitoring optical waveguide for reflecting light propagated along said monitoring optical waveguide, the width of a reflection face of said reflecting portion being substantially equal to the cut-out width of said monitoring optical waveguide.