An optical circuit device includes a directional coupling section and at least one diffraction grating. The directional coupling section is comprised of first and second optical waveguides and having a length n times (n is an integer equal to or larger than two) the minimum length required to completely couple light inputted to the first optical waveguide with the second optical waveguide. The diffraction grating is formed in the directional coupling section and has a specific reflection characteristic. When input light beams having different wavelengths are inputted to the input side of the first optical waveguide, light having a reflection wavelength in the diffraction grating is demultiplexed/outputted to the input side of the second optical waveguide. Light beams having other wavelengths are outputted to the output side of the first or second optical waveguide. When the light having the reflection wavelength in the diffraction grating is inputted through one of the output sides of the first and second optical waveguides from which the light beams having other wavelengths are not outputted, the light having the reflection wavelength input through the output side of the optical waveguide is multiplexed with the output light beams having other wavelengths.