To provide an observation optical system and an image forming optical system having a wide observation viewing angle and a short focal distance and for forming an intermediate image once by mutually compensating eccentric aberration by two eccentric prisms.

The system is provided with a 1st prism 10 and a 2nd prism 20 which is positioned on the image side of the prism 10, and the 1st prism 10 is provided with 1st to 4th surfaces 11 to 14, the 1st surface 11 functions as an incident surface and also as a reflection surface, the 2nd surface 12 functions as a reflection surface, the 3rd surface 13 functions as a reflection surface and the 4th surface 14 functions as an outgoing surface, and at least one of the 1st and 2nd surfaces 11 and 12 is constituted of a rotation-asymmetrical surface for applying a power on a luminous flux and also compensating the aberration caused by the eccentricity. And, the 2nd prism 20 is provided with a reflection surface 22, an incident surface 21 and an outgoing surface 23, and also the reflection surface is constituted of a rotationasymmetrical surface for applying a power on the luminous flux and also compensating the aberration caused by the eccentricity, and an intermediate image field 4 is formed between the 1st surface 11 of the 1st prism 10 and the outgoing surface 23 of the 2nd prism 20.