In an observation using a microscope, it is necessary to observe a sample in a natural color, so that an objective lens with highly corrected chromatic aberration has been expected. Moreover, in order to make the handling of the sample easy, an objective lens keeping a working distance as long as possible has been expected. However, when a long working distance is to be secured, the object side focal length of the objective lens has to be long, and a negative lens group having strong refractive power has to be disposed to the image side, so that chromatic aberration naturally becomes worse. Accordingly, recently, there have been proposed various objective lenses using a diffractive optical element in order to highly correct chromatic aberration (see, for example, Japanese Patent Publication No. 3312057).
However, the objective lens whose chromatic aberration is highly corrected has had insufficient working distance for industrial use, and large undulation in spherical aberration and coma.