A projection optical system for projecting an image on a surface is provided. The image is an enlarged image of an image which is formed on an image forming element. The projection optical system includes a coaxial optical system having an optical axis; and a non-coaxial optical system including a rotationally asymmetric curved-surface mirror. The non-coaxial optical system does not share the optical axis with the coaxial optical system. The coaxial optical system includes a first lens having a positive refractive power and being an aspheric plastic lens; and a second lens having a negative refractive power and being an aspheric plastic lens. The first lens has a first refractive index distribution, and the second lens has a second refractive index distribution.