Disclosed is an optical system for a head-mounted display, in which image rays provided from a display device are diverged to form and provide an enlarged image. First, second, and third lenses or first, second, third, and fourth lenses are sequentially disposed from an object along an optical axis and provide the enlarged image on an image plane, and angles of principal rays are gradually increased from the optical axis toward an edge of the image plane. Thus, as the angles of principal rays increase, the angle of view is improved to provide a large image so that it is possible to appreciate a vivid image due to an increase in the sense of presence and the degree of involvement. The size of the entire optical system can be reduced, thus a head-mounted display can be made compact and lightweight.