An optical waveguide device having a Mach-Zehnder type waveguide formed on a substrate is provided in which a slope of two waveguides input to an optical coupler on an output side of the Mach-Zehnder type waveguide is 0 degrees, a waveguide of the optical coupler after being coupled by the optical coupler is a multi-mode waveguide, and the waveguide which is output from the optical coupler is a three-branched waveguide including an output main waveguide and two output sub waveguides interposing the output main waveguide therebetween.