Provided is a projection optical system. The projection optical system includes: A projection optical system comprising: a first optical part that comprises at least one optical lens, and projects and displays light beams; a second optical part that comprises an image forming element and at least one optical lens, and emits incident light beams as light beams with images; and a third optical part that comprises a light source unit and a deflector that is located between the first and second optical parts, and directs light beams emitted from the light source unit to the second optical part and directs the light beams with images emitted from the second optical part to the first optical part.