A projection optical system (10) projects from a first image plane (5) on a reducing side to a second image plane (6) on an enlargement side. The projection optical system (10) includes a first optical system (11) that includes a plurality of lenses and forms a first intermediate image (51) formed inside the first optical system (11) by light incident from the reducing side into a second intermediate image (52) on the enlargement side of the first optical system (11); a second optical system (12) that includes a first reflective surface (M1) with positive refractive power which is positioned further to the enlargement side than the second intermediate image (52); and a glass block (30) that is disposed between the first optical system (11) and the first reflective surface (M1), the glass block (30) passing light rays from the first optical system (11) to the second intermediate image (52).