To minimize both of astigmatism and the curvature of an image plane, to enlarge an allowable range when a triplet lens is manufactured and used and to enable the triplet lens to be easily handled by forming zonal structure having the action of a blazed positive diffraction lens on either surface of one lens of the triplet lens.

This triplet lens is constituted of a first positive lens 10, a second negative lens 20 and a third positive lens 30. Then, at least one lens surface is provided with the zonal structure having the action of the blazed positive diffraction lens. A normal lens is provided with such a characteristic that it is provided with the curvature of an image plane being the curvature of Petzval's sum P decided by P=Σ/n by defining refractive index as n and the power of the lens surface as . However, the diffraction lens is provided with such a characteristic that the curvature of an image plane is not caused regardless of the power of the lens. Thus, both of the astigmatism and the curvature of the image plane can be minimized by approximate the Petzval's sum to zero by alotting the positive power to the diffraction lens and making the power of the positive lens of the diffraction lens as small as the allotted amount or making the power of the negative lens large.