To achieve a wide angle of view and a compact structure in an objective optical system for an endoscope, the objective optical system having in an optical system a prism for converting the direction of a field of view and used for an oblique view or side view.

The objective optical system for the endoscope includes, in order from the object side, a negative first group, a diaphragm, and a positive second group. An optical member for converting the direction of a field of view is disposed between the first and second groups. The optical system that is a wide angle and compact can be obtained by satisfying formulas (1): -0.8COPYRIGHT: (C)2008,JPO&INPIT