A projection optical system includes a first optical system, a second optical system, and first and second reflection mirrors. The first and second reflection mirrors are arranged on an optical path between an image forming element and a reflection surface of the second optical system, so that a direction of an optical path of light beams from the second reflection mirror to the reflection surface contains a component of a direction opposite to a predetermined direction from the image forming element to the first optical system, and a projection image of the image forming element on a conjugate surface, a projection of the reflection surface onto the conjugate surface, and a projection of the image forming element onto the conjugate surface are arrayed in this order on the conjugate surface.