[Problems] In the objective optical system, and wide field angle while suppressing the occurrence of the chromatic aberration of magnification. In order from A object side, a first lens group having negative refractive power (G1), the diaphragm (St), is arranged a second lens group having positive refractive power (G2), the first lens group ( G1) includes, in order from the object side, a first lens which is a negative single lens (L1), it is assumed to be formed by placing a cemented lens having a negative refractive power which is formed by joining a positive lens and a negative lens (LS1) The second lens group (G2) includes, in order from the object side, a positive fourth lens is a single lens (L4), a positive lens and a negative lens are bonded to become a positive cemented lens having a refractive power (LS2) It is assumed to be formed by arranging the conditional expression (1): 15.0
