PURPOSE: To easily adjust the focusing of a direct and a reflected image by providing a power correcting lens to either of the 1st and the 2nd optical path so that the lens is movable in the direction of the optical axis.

CONSTITUTION: A right-angled prism 9, a convex lens 10 as the power correcting lens, and a right-angled prism 11 are provided on the optical path of the 1st luminous flux 5 projected from a cemented prism 8, and a right-angled prism 12 is provided on the optical path of the 2nd luminous flux 6 projected from the cemented prism 8. The direct image B is larger in magnification than the reflected image B because of a difference in object distance, so the convex lens 10 is used as an optical reduction system so as to correct the difference in magnification and form the direct image A and reflected image B of the same size, and movable along the optical axis 1b. The right-angles prism 13 is so combined that the luminous flux 5 from the right-angled prism 11 and the 2nd luminous flux 6 from the right-angled prism 12 adjoin to each other about the ridge line of the right-angles prism 13, thereby allowing the direct image B and reflected image B formed by those pieces of luminous flux to meet together in one visual field 14.