An endoscope includes a circular cylindrical member, a distal end member, and an image forming optical system. The circular cylindrical member has an inner peripheral surface and an outer peripheral surface. A space between the inner peripheral surface and the outer peripheral surface is filled with a transparent material. The distal end member is positioned at one end of the circular cylindrical member. The image forming optical system is disposed at an interior of the circular cylindrical member. The image forming optical system includes only transmitting surfaces and all the transmitting surfaces are disposed such that a normal of a plane at a point intersecting with the optical axis is aligned with the optical axis. The image forming optical system has a curvature of field, and the following conditional expression (1) is satisfied:−；10<；P′；<；−；0.8 ； ；(1).
