There is provided an objective optical system that is easy to produce, capable of forming optical images that can be observed without difficulties while small in size. The objective optical system includes, in order from the object side, a flat plate OP1, an aperture stop S, and a substantially hemispherical planoconvex lens L1 having an outer diameter substantially equal to the outer diameter of the flat plate. The flat plate and the planoconvex lens L1 are cemented together with the aperture stop between. The objective optical system satisfies the following conditional expressions (1), (2), and (3): 0.3