[Problems] In the objective optical system, and wide field angle while suppressing the variation and aberrations of the angle of view caused by falling or the eccentricity of the lens surface due to manufacturing of the circumstances related to the first lens group of the front aperture. A in order from the object side, a negative first lens group (G1), the diaphragm (St), is arranged a positive second lens group (G2), the first lens group (G1) includes, in order from the object side a negative first lens is a single lens (L1), and those composed by placing a negative cemented lens formed by joining a positive lens and a negative lens (LS1), a second lens group (G2), the object-side in order from the positive fourth lens is a single lens (L4), the positive lens and it is assumed to be formed by arranging a negative lens are bonded to become a positive cemented lens (LS2), the conditional expression (1): - 1. 5
