PURPOSE: To obtain an optical system which reduces cost, prevents the intrusion of dirt and has high accuracy by forming optial members such as a Fresnel lens, split image, etc. into one body thereby constituting a pentagonal prism.

CONSTITUTION: A Fresnel lens face 3 and a split image 4 are formed similtaneously of a plastic under pressure on the light incident face 2 of a pentagonal prism body 1. Since both are formed at one time by the integral forming, the intrusion of dirt between the prism 1 and the Fresnel lens face is prevented and since the prism and lens are unaffected by sinkage, the optical system having high accuracy is obtd.