The present disclosure is directed to an optical system having a reduction conjugate point on a reduction side and a magnification conjugate point on a magnification side that are optically conjugate with each other, and includes a prism including a first transmission surface located on the reduction side, a second transmission surface located on the magnification side, and at least three reflection surfaces located on an optical path therebetween, wherein in the meridional plane, two light rays traveling in a direction perpendicular to the first rectangular region from two points on the first rectangular region intersect at two intersection positions after passing through the first transmission surface and then reflected by the at least three reflection surfaces and before passing through the second transmission surface, and the number of reflection of the two light rays intersecting at each intersection position before reaching the respective intersection positions is the same.