An image forming optical system (21) is an image forming optical system wherein an object plane (P1) and an image plane (P2) are inclined to each other in mutually opposite directions, with respect toa first axis (25) connecting a point in the object plane (P1) and a point in the image plane (P2) conjugate with the point in the object plane, in a flat plane which includes the first axis and whichis perpendicular to an intersection line between a plane including the object plane and another plane including the image plane, the image forming optical system including a first optical member (51)which is non-coaxial with respect to the first axis in the flat plane. In a tilted-lens image forming optical system, it is possible to suppress the lowering of imaging performance.