Provided is an objective optical system comprising, in order from an object side: an aperture stop a positive first group a second group a positive third group and a fourth group, wherein the first group is formed of a single meniscus lens or plano-convex lens, whose convex surface faces an image side, the second group is formed of a single lens, the third group is formed of a cemented lens consisting of a positive lens and a negative lens, the fourth group is formed of a single lens, and Conditional Expressions (7) and (8) below are satisfied, where f is a focal length of an entire system, f4 is a focal length of the fourth group, and f2 is a focal length of the second group.−0.5
