Element fixing mounting portions are formed in the shape of bridge beams on both the side surfaces or the like of the first optical surface, which is disposed in confrontation with an image display element, of a Free-Form-Surface prism 61 acting as an optical element, the side surfaces being located externally of the effective light beam range of the first optical surface. Then, the optical element can be mounted on a cabinet making effective use of a space by projecting positioning bosses or the like from the mounting portions, the bosses being used when optical element is mounted on the cabinet.