PURPOSE: To obtain a compact and inexpensive image forming optical system, by providing a cylindrical mirror and a cylindrical lens orthogonal to the cylindrical mirror and satisfying the prescribed conditions for formation of an image.

CONSTITUTION: A prescribed space is secured between a cylindrical mirror 2 set at the back of a lens 1 and a cylindrical lens 5 which is orthogonal to the mirror 2 so that coincidence is obtained for the center of curvature between them. A subject 3 is formed as an erecting real image 3' in the direction of the section X having the cruvature of the mirror 2 and in the combination of a recurrent reflection system and the lens 1. While a subject 4 is formed as a reverse real image 4' in the direction Y rectangular to the direction X. The aberration of the X and Y directions is compensated by the lens 5. This image forming device forms an erecting image in the direction X and a reverse image in the direction Y. Thus these plural small-sized optical systems are converted to an array in the direction X. As a result, a compact and long image forming device is obtained.