An objective lens OL comprises: a first lens group G1 disposed on an object side, having positive refractive power as a whole and having a positive lens (plano-convex lens L1) which is disposed closest to the object and of which lens surface closest to the object is a plane or a surface having a low curvature, and at least one cemented lens (cemented lens CL12 or the like); a second lens group G2 disposed on an image side, having negative refractive power as a whole, and having a concave surface facing the image and a concave surface facing the object, which face each other; and a diffractive optical element GD in which two diffractive element constituents made from different optical materials are cemented, which has a diffractive optical surface D formed with diffraction grating grooves on the cemented surface, and which is disposed closer to the object than a position where a principal ray crosses the optical axis.