To reduce an insertion loss and polarization dependency, to widen a wavelength band width, to compactify a size, and to reduce a cost.

A grating used for this multiplexer is constituted to make incident light to each groove reflective, to make emitted lights from the respective grooves intensified each other in a direction of the incident light by an interference effect, to make a wave front of an evanescent wave in the groove parallel to a normal direction of the grating, and to make a migration of the evanescent wave in the groove matched by a groove tube. High diffraction efficiencies are obtained thereby in both TM polarization and TE polarization even in the diffraction order not more than several order, the wavelength band width is widen resultingly, and the polarization dependency is allowed to get small without using a compensation means.