The invention provides a liquid-immersion objective optical system including a first lens group and a second lens group. The first lens group includes a first lens component having positive refractive power, an object side thereof being a flat surface and a convex surface thereof facing an image-plane side; a second lens component having positive refractive power, a convex surface thereof facing the image-plane side; a third lens component having positive refractive power as a whole, formed by cementing a biconvex positive lens and a negative lens; and a fourth lens component formed by cementing a negative lens and a biconvex positive lens. The second lens group includes a fifth lens component formed by cementing a lens having a concave surface facing the object side and a lens having a convex surface facing the image-plane side; a sixth lens component having positive refractive power; and a seventh lens component having negative refractive power. The focal length F1 of the entire first lens group, the magnification M of the objective optical system, the distance LT between the object plane and the image plane, and the distance LG between the object side of the first lens component L1 and the image side of the fourth lens component L4 satisfy the following conditional expressions:

0.2|MF1/LT|0.45

0/2|LG1/LT|0.4.