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, and projects the enlarged image, the projection optical system being a fixed focal length lens or a zoom lens having, 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, characterized in that positive lenses (Mp1 and Mp2) are positioned on each of the enlargement side and a reduction side of the intermediate image so as to be adjacent to each other and are both positive meniscus lenses concave on the intermediate image side, in a case of the fixed focal length lens, following conditional expressions and are satisfied, and in a case of the zoom lens, the following conditional expressions and are satisfied at a wide angle end: 0.4 < Tm 1 / Tw < 0.7 T 12 w / Tw < 0.2 where definitions are provided as follows: Tm1: an axial distance from a lens surface closest to the enlargement side to a lens surface closest to an intermediate image side, in the first optical system, Tw: an axial distance from the lens surface closest to the enlargement side in the first optical system to a lens surface closest to the reduction side in the second optical system, and T12w: an axial air-gap between the positive lenses (Mp1 and Mp2) positioned on each of the enlargement side and the reduction side of the intermediate image so as to be adjacent to each other. the projection optical system being a fixed focal length lens or a zoom lens having, 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, characterized in that positive lenses (Mp1 and Mp2) are positioned on each of the enlargement side and a reduction side of the intermediate image so as to be adjacent to each other and are both positive meniscus lenses concave on the intermediate image side, in a case of the fixed focal length lens, following conditional expressions and are satisfied, and in a case of the zoom lens, the following conditional expressions and are satisfied at a wide angle end: 0.4 < Tm 1 / Tw < 0.7 T 12 w / Tw < 0.2 where definitions are provided as follows: Tm1: an axial distance from a lens surface closest to the enlargement side to a lens surface closest to an intermediate image side, in the first optical system, Tw: an axial distance from the lens surface closest to the enlargement side in the first optical system to a lens surface closest to the reduction side in the second optical system, and T12w: an axial air-gap between the positive lenses (Mp1 and Mp2) positioned on each of the enlargement side and the reduction side of the intermediate image so as to be adjacent to each other. Tm1: an axial distance from a lens surface closest to the enlargement side to a lens surface closest to an intermediate image side, in the first optical system, Tw: an axial distance from the lens surface closest to the enlargement side in the first optical system to a lens surface closest to the reduction side in the second optical system, and T12w: an axial air-gap between the positive lenses (Mp1 and Mp2) positioned on each of the enlargement side and the reduction side of the intermediate image so as to be adjacent to each other.