Abstract: The objective lens is a biconvex plastic lens having first and second surfaces which are both aspheric. The objective lens is divided into an inside area which is inside of a circle of an effective diameter of the objective lens and an outside area which is outside of the circle of the effective diameter. A diffractive lens structure having a plurality of concentric ring-shaped steps is formed in the inside area. A surface of the outside area is continuous with no steps and corresponds to a macroscopic base curve of the inside area. The objective lens has a gap of a spherical aberration between the inside area and the outside area.