Projection optical system and projector

A projection optical system that projects from a first image plane on a reducing side onto a second image plane on a magnifying side, comprising:

• a first optical system that includes a plurality of lenses and forms a first intermediate image formed inside the first optical system by light that is incident from the reducing side, into a second intermediate image on the magnifying side of the first optical system, the first intermediate image being formed on a first side of an optical axis and the second intermediate image being formed on a second side of the optical axis;
• and
• a second optical system including a first reflective surface with positive power located on the magnifying side of the second intermediate image,
• wherein the first optical system includes:
• an intermediate lens, the first intermediate image being formed so as to be inclined so as to approach the reducing side as a distance from the optical axis increases and to cross the intermediate lens;
• a first lens group that includes the intermediate lens and multiple additional lenses, wherein the multiple additional lenses of the first lens group are located on the reducing side of the intermediate lens;
• and
• a second lens group that is located on the magnifying side of the intermediate lens, and
• a combined focal length fa of the projection optical system and a combined focal length f2 of the second lens group satisfy the following condition
• 8<f2/fa<20.