PURPOSE: To provide an aspheric single lens by which a numerical aperture becomes large and the work is facilitated.

CONSTITUTION: An aspheric single lens has both convex surfaces whose both first and second surfaces are composed of the aspheric surfaces. When a paraxial radius of curvature of an (i) surface is denoted by (r1) and an axial thickness is denoted by (d) and a refractive index is denoted by (n) and a focal distance is denoted by (f) and a numerical aperture is denoted by NA and a dislocation quantity from a reference spherical surface of an aspheric surface in an effective diameter of a first surface is denoted by (), respective conditional expressions of [-0.2<r1/r2<0...(l), 0.1<d/f.n<0.3...(2) and 0.02<{(n-1)3/NA4}×(Δ1/f)<0.10...(3)] are satisfied, and when a distance up to a tangent plane of the aspheric surface apex from an optional point on the aspheric surface is denoted by Z and a distance up to the optical axis from an optional point is denoted by (h) and a conical constant of the (i) surface is denoted by (Ki) and aspheric surface factors of the (i) surface are denoted by Ai and Bi, the aspheric surface is expressed by the following developing expression of [Z=(h2/ri)/[1+-{1-(1+Ki)h2/ri2}1/2]-+Aih4+Bih6...(4)].