An objective optical system for endoscope includes in order from an object side, a first group having a positive refractive power, an aperture stop, and a second group having a positive refractive power, wherein the first group includes a first lens having a negative refractive power, of which an object side is a flat surface, and a second lens having a positive refractive power, the second group includes a cemented lens, and the cemented lens includes a third lens having a positive refractive power and a fourth lens having a negative refractive power, and the following conditional expressions (1), (2), (3), and (4) are satisfied: 0.13≤La/Lb≤0.17 −2.7≤f4/f≤−2.2 −0.7≤ne2×f2/ne4×f4≤0 and 2.5≤νd3/νd4≤3.5
