To realize an objective optical system, which enables a user to observe at a wide angle and has less blind spots, with a small-sized objective optical system.

The objective optical system is provided with a hemispherical transparent member, a first reflection face which reflects forward a luminous flux transmitted through the transparent member, a second reflection face which reflects backward a reflected luminous flux reflected by the first reflection face, and a lens group which forms an image of a reflected luminous flux reflected by the second reflection mirror, and the inside and outside faces of the transparent member and the first and second reflection faces are formed with faces of revolution which have the optical axis of the image forming lens group as the axis of revolution.