PURPOSE: An optical system of a pico-projector is provided to form a curvature convex toward the outside on a second and a third surface of a prism structure and increase reflection efficiency, thereby improving image quality. CONSTITUTION: An internal reflection structure (120) includes one piece of tilt relay lens (121) and one piece of prism structure (122). The tilt relay lens is positioned in a slanted way corresponding to a first surface of the prism structure. A second surface (122b) of the prism structure, facing a digital mirror device (DMD) element (130), has a convex curvature toward the DMD element. A third surface (122c) of the prism structure, facing a projection optical part (140), has a convex curvature toward the projection optical part.
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