PURPOSE: To provide a microscopic objective lens which is as bright as ≥0.2 in NA, is as small in the number of lens elements as about three elements and is capable of well correcting aberrations by having one element of specific radial type distributed index lens in a lens system.

CONSTITUTION: This microscopic objective lens has one element of the radial type distributed index lens (third lens) having a refractive index distribution in a radial direction from the optical axis expressed by equation I in the lens system. The distributed index lens satisfies the conditions of equation II. In the equations I, II, r denotes the distance in a radial direction from the optical axis, N(r) denotes the refractive index at the point of the radius r, N00 denotes the refractive index on the optical axis, N10, N20... respectively denote quadratic, four dimensional ... coeffts., V00, V10 denote the dispersion of the radial type distributed index lens and are expressed by V00=(N00d-1)/(N00F-N00C), V10= N10d/(N10Fd-N10C). Where, N00d, N00F, N00C respectively denote the refractive indices on the optical axis to d line, F line and C line, N10d, N10F, N10C respectively denote refractive index distribution index coefft. N10 to the d line, F line and C line.