An objective optical system comprising a lens group that forms an object image, a polarizing beam splitter having an optical path dividing surface that divides an optical path into a first optical path that intersects the optical axis of the lens group and a second optical path that extends straight on the optical axis, a first reflection surface provided in the first optical path, a second reflection surface provided in the second optical path, and a quarter wave plate provided between the polarizing beam splitter and the first reflection surface. The objective optical system satisfies the following conditional expression (1):


(Tp+Ts)/(100−Ts)≥1.25  (1)

where Tp (%) is the transmittance of the optical path dividing surface for p-polarized light, and Ts (%) is the transmittance of the optical path dividing surface for s-polarized light.