An objective optical system includes a first group having positive refractive power and a piano-convex lens with the convex surface facing the image side; a second group having positive refractive power and a lens whose extreme-object-side lens surface is a convex surface facing the object side; a third group having negative refractive power and a lens whose extreme-image-side lens surface is a concave surface facing the image side; a fourth group having positive refractive power and a lens disposed on the extreme object side, whose image-side lens surface is a convex surface facing the image side and a lens disposed on the extreme image side, whose object-side lens surface is a convex surface facing the object side; and a fifth group having positive refractive power and a combined lens formed by joining a convex lens and a concave lens, the joined surface having negative refractive power.