To provide a small-sized endoscopic optical system giving a clear image with less distortion even in a wide viewing angle.

This endoscopic optical system is provided with a prism member 3 having reflection surfaces 5, 6 for bending an optical path, an objective optical system provided with the prism member 3 for forming an object image and an image transmission system to guide the object image formed by the objective optical system to an observing device along the major axial direction. This system has a shape to cause eccentric aberration by bending the optical path, and the prism member 3 is constituted of a non-rotary symmetric plane shape having at least one surface of curved surfaces 4-7 having an optical action on the optical path, and having no rotary symmetric axis in an inside surface and an outside surface also so that the curved surface corrects the eccentric aberration.