PURPOSE: To provide a large numerical aperture and a long working distance, by constituting a regenerative lens with four groupa of lenses, namely, a positive lens, a negative lens, a positive lens, and a positive meniscus lens having the concave in the disc side and satisfying specific conditional inequalities.

CONSTITUTION: The first-group lens of a positive lens, the second-group lens of a negative lens, the third-group lens of a positive lens, and the fourth-group lens of a positive meniscus lens having the concave in the disc side are arranged from the side of a light source in order and satisfy conditional inequalities (1)W (4). On a basis of conditional inequality (4), the image-side (disc-side) surface of the first-group lens and the object-side (light source-side) surface of the second- group lens are brought into contact with each other to shorten an air interval d2 between these lenses, and thus, the synthetic focal length between the first- group lens and the second-group lens is increased to make the working destance longer. Since corrections for spherical aberration, coma, and astigmatism are required when the working distance is made longer, lenses are constituted to satisfy conditional inequalities (1)W(3).