There is provided a projection optical system (1) that projects from a first image plane on a reducing side to a second image plane on an enlargement side, including a first refractive optical system (11) that includes eight lenses (L1) to (L8) and forms a first intermediate image (31) on the enlargement side using light that is incident from the reducing side, a second refractive optical system (12) that includes six lenses (L9) to (L14) and forms the first intermediate image (31) on the reducing side into a second intermediate image (32) on the enlargement side, and a first reflective optical system (20) that includes a first reflective surface (21a) with positive refractive power that is positioned closer to the enlargement side than the second intermediate image (32).