The invention relates to a viewing and an image pickup optical system for display devices, which can be used with high efficiency at a plurality of wavelengths and enables bright images to be viewed with satisfactory color reproducibility and well-corrected aberrations. The optical system comprises a first prism 3, a second prism 4 and a volume hologram element 6 disposed between them and cemented to them. The hologram element 6 comprises a first volume hologram optimized in such a way as to effect diffraction at least at a first wavelength and a second hologram optimized in such a way as to effect diffraction at a second wavelength different from the first wavelength. The first and second holograms forming the hologram element 6 are identical with each other in terms of the shape and spacing of interference fringes on their surfaces but different from each other in terms of the spacing and tilt of interference fringes in their hologram media.