An optical device includes an optical fiber mount with an optical fiber disposed thereon, and an optical demultiplexer having a slit defined in the optical fiber across an optical axis thereof, and an optical demultiplexing member inserted in the slit for demultiplexing a portion of an optical signal beam transmitted through the optical fiber, wherein the optical demultiplexer guides a demultiplexed optical signal beam from the optical demultiplexing member out of the optical fiber, an optical path changer for changing an optical path of the demultiplexed optical signal beam guided out of the optical fiber, a waveguide serving as a medium from which the demultiplexed optical signal beam is emitted to the optical path changer, and a filter disposed on a surface of the waveguide.