A projection optical system is disclosed. The projection optical system includes a first optical system configured to form a first image conjugated with an object and a second optical system configured to project a second image conjugated with the first image toward a projection surface. At least one of the first optical system and second optical system includes at least one optical element(s) movable relative to the object is provided. An image distance of the projection optical system is changed and a size of the second image is changed, by moving at least one of the optical element(s) relative to the object.