Provided is an objective optical system composed of, in order from an object side, a first and a second groups, the first group including: a positive and single lens having a flat surface on an object side and a convex surface directed toward an image plane; a positive and single lens having a convex surface directed toward an object; a positive lens in which a convex lens having a flat surface directed toward the object and a negative lens are cemented, and the second group including: a lens in which a negative lens and a double-convex lens are cemented; a lens in which a lens having a concave surface directed toward the object and a lens having a convex surface directed toward the object are cemented; a negative lens in which a negative lens, a positive lens, and a negative lens are cemented; and a positive and compound lens.