A projection optical system which has a short projection distance and a compact structure, and allows oblique projection. The projection optical system projects luminous flux from an image forming element for forming an original image onto a projection surface which is oblique to a central principal ray which is a principal ray of luminous flux traveling from the center of the original image to the center of a finally formed image. The system includes a plurality of reflecting surfaces each having a curvature. In addition, the projection optical system satisfies a predetermined expression (1).