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.
