An optical device includes a first waveguide that inputs first signal light with a first optical characteristic, and a first convertor that converts the first signal light that travels from the first waveguide into second signal light with a second optical characteristic. The device includes an optical circuit, when the converted second signal light passes through the circuit, performs first optical processing on the second signal light. The device includes a second convertor that converts the second signal light that travels from the circuit and that is subjected to the first processing into third signal light with the first characteristic. The optical device includes the circuit that, when the converted third signal light passes through the circuit, performs second optical processing on the third signal light, and a second waveguide that outputs the third signal light that travels from the circuit and that is subjected to the second processing.