To provide a diffraction element which can be used for a plurality of wavelength bands and also to provide an optical multiplexing and demultiplexing device which uses effects of the diffraction element.

The diffraction element has a transparent substrate (2) which has 1st and 2nd surfaces, a periodic groove which is engraved on the 1st surface, a reflecting film (1) which is provided thereupon, and a reflection preventive film (4) which is provided on the 2nd surface and the multiplexing and demultiplexing device which has the diffraction element is equipped with a 1st light input device (10) which makes light incident on a 1st incidence surface (on the side of the 1st surface where the reflecting film is provided) of the diffraction element, a 1st light reception device (12) which receives the light made incident on the 1st incidence surface and diffracted, a 2nd light input device (11) which receives the light made incident on the 1st incidence surface and reflected and makes it incident on the 2nd incidence surface (the surface where the reflection preventive film is provided) of the diffraction grating, and a 2nd light reception device (13) which receives the light made incident on the 2nd incidence surface and diffracted.