A projection optical system that enlarges an image to be displayed on an image display surface to an angle of view of 70 degrees or more, projects the enlarged image, and includes: a fixed focal length lens that has, in order from an enlargement side, a first optical system and a second optical system on a same optical axis, wherein the second optical system forms an intermediate image of the image, the first optical system enlarges and projects the intermediate image, the projection optical system comprises positive lenses that are positioned on both of the enlargement side and a reduction side of the intermediate image and are adjacent to each other, and following conditional expressions are satisfied: 0.4