There is provided an objective optical system for endoscope, which is small-sized and has a favorable optical performance.The objective optical system for endoscope includes in order from an object side, a first negative lens which is planoconcave, a second meniscus lens having a convex surface directed toward an image side, a third positive meniscus lens having a convex surface directed toward the object side, a positive lens unit, and a cemented lens in which, a positive lens and a negative lens are cemented. Focusing is carried out by moving the third positive meniscus lens along an optical axis, and the second meniscus lens is a negative lens, and the objective optical system for endoscope satisfies the following conditional expression (1), and satisfies at least one of the following conditional expressions (4-1) and (4-2).2.3≦fp/Ih≦5 (1)−1.5≦r2/r3≦−0.28 (4-1)−0.2≦r2/r4≦−0.08 (4-2)where,fp denotes a focal length of the positive lens unit, andIh denotes the maximum image height of the objective optical system for endoscope,r2 denotes an image-side radius of curvature of the first negative lens,r3 denotes an object-side radius of curvature of the second meniscus lens, andr4 denotes an image-side radius of curvature of the second meniscus lens.
