To obtain a good wavefront aberration at a parallel wavefront to a point of an infinite object and a good image formation efficiency aberration to a finite object by specifying a shape of each of three lenses of a group and constituting an image formation lens to satisfy a predetermined conditional expression, and obtain good interference fringe images when the lens is used in an oblique incidence interferometer apparatus.

A first lens L1 of a negative meniscus lens having a concave face directed to an object, a second lens L2 of a negative meniscus lens having a convex face directed to the object, and a third lens L3 of a double convex lens having a face of a large curvature directed to the object are arranged sequentially from the side of the object. A luminous flux entering along an optical axis X from the side of the object forms an image on an image formation face. When a thickness of the first lens L1 is dF, a focal length of the first lens L1 is f1, a focal length of the entire system is (f) and a lens length of the entire system is (d), the following two conditional expressions are satisfied. 0.1<f<f1<0.6 and 0.3<dF/d<0.6.