! diffraction device having uneven diffraction gratings on two surfaces, from which at least two separate light beams can be taken out in the same direction, and not greately changing the propagating direction of the diffracted light even if the temperature of the operating environment changed. One diffraction grating of uneven sectional form is provided on the input surface of a transparent substrate, and two diffraction gratings of uneven sectional form are provided on the output surface. The grating pitch of the first one is equal to the grating pitch of one of the second ones. In addition, a reflective diffraction device which is not considerably dependent on the direction of polarization of the input light, but exhibits a wavelength dependence of excellent diffraction efficiency is provided. A pseudo sawtooth−like diffraction grating is provided on one surface of a transparent substrate, a light reflective film is formed on a diffraction grating part, and an antireflection film is formed on the surface on the opposite side.
