PURPOSE: To miniaturize the titled optical system and to contrive the reduction of cost by constituting a first reflecting surface group for reflecting the reflected light of a multipath reflecting surface, and a second reflecting surface group for reflecting the reflected light of the first reflecting surface group and making it incident on the multiplex multireflecting surfaces, so that each reflected light can form a closed polygon.

CONSTITUTION: An incident point in a multiplex reflecting surface 1 is set to only one place, and on the other hand, the first reflecting surface groups 2, 21 and 22 on which the reflected light of each time of the multiplex reflecting surface is made incident are provided by the number of times of reflection in the multiplex reflecting surface 1, and also, a second reflecting surface groups 3, 31 and 32 for making the reflected light of the first reflecting surface groups 2, 21 and 22 incident on the multiplex reflecting surface 1 are provided. Therefore, although the optical path length is long, the whole volume and the size of the system do not become so large. According to such constitution, the image forming optical system can be miniaturized and the reducing of cost can be realized.