1. An image-forming optical system which forms an image of an object, said image-forming optical system comprising: at least one prism member, wherein said prism member has a first surface, a second surface, a third surface, and a fourth surface, wherein said first, second, third and fourth surfaces face each other across a prism medium, so that light from an object side of said prism member enters said prism member by passing through said first surface and is reflected by said second surface and further reflected by said third surface, and the reflected light exits from said prism member by passing through said fourth surface, and wherein at least one of said second surface and said third surface has a surface configuration formed from a rotationally asymmetric curved surface.