To provide an optical multiplexer and an optical demultiplexer for a dense wavelength division multiplex(DWDM) fiber optical communication system.

As the multiplexer, a device functions so as to spatially couple optical signals from plural laser sources (respectively having different wavelengths) and to radiate a spatially coupled laser beam to a single optical fiber. As the demultiplexer 15, the device functions so as to spatially divide the different wavelengths of a wavelength division multiplex optical link and to respectively radiate the different wavelengths to separate optical fibers. Even in either enforcement form, the device contains both of a bulk optical parts and an integrated optical parts. The spatial couplement or spatial separation of the laser beams having different wavelengths is attained by using a bulk diffraction grating 27.