PURPOSE: To provide a beam splitting prism for limiting the increasing of the size and the weight of the prism accompanying the increase of a number of emitted light beams and capable of increasing the ratio of common use of tools and jigs.

CONSTITUTION: This prism is composed of at least three prism member 1-3, a first prism memeber 1 on which a light beam from an image forming optical system is made incident first, has an incident plane 1a perpendicular to the optical axis of the image forming optical system, a first reflection plane penetrating by the optical axis of the image forming optical system and a second reflection plane having a nearly the same angle made with the first reflection plane and is adjacent to the first reflection plane, after a part of the light beam made incident on the first prism member 1 is successively reflected by the first reflection plane, the incident plane and the second reflection plane, it is emitted from the incident plane almost perpendicularly. At this time, a second prism member 2 is arranged opposing to the second reflection plane of the first prism member 1.