To provide a variable magnification optical system capable of projecting an image onto an image-projected surface with a sufficient magnification without requiring a large-sized mirror.

The variable magnification optical system 100 is configured to pass a luminous flux emitted from an object surface 13 through a first optical system 11, a second optical system 12 and a reflective optical element 18 having a magnification power in this order, and project the image 16 onto the image-projected surface. A magnification of the image 16 projected on the image-projected surface is changed by moving the reflective optical element 18, thus changing a distance between the image-projected surface and the reflective optical element 18, while an incident angle of the luminous flux on the image-projected surface from the reflective optical element 18 is maintained substantially constant.