A projection optical system that receives light from a display element to project an image displayed by the display element onto a screen with enlargement at a varying magnification achieved by varying the projection distance to the screen has: a refractive optical system composed of one or more refractive lenses and having a positive optical power; and a concave-surfaced mirror disposed to the screen side of the refractive optical system and having a plane-symmetric reflective surface. The projection optical system includes at least one optical element designed to be movable for focusing. The projection optical system forms an intermediate image between the refractive optical system and the concave-surfaced mirror. Moreover, a prescribed conditional formula is fulfilled.