An optical projection system for projecting an enlarged image on a projection surface is provided. The optical projection system includes an image forming element, a coaxial optical system and a concave mirror. The coaxial optical system and the concave mirror are arranged in this order on an optical path from the image forming element to the projection surface. The coaxial optical system includes lens groups and an aperture stop that share an optical axis. The lens groups include a first lens group and other lens groups. The first lens group has negative refractive power and independently moves in an optical axis direction for adjusting the focus of the optical projection system. The aperture stop is arranged at a position closer to the image forming element than the first lens group that is arranged closest to the concave mirror among the lens groups having the negative refractive power.