The invention relates to an image-formation optical system that meets both demands for high performance and compactness, and an imaging system that incorporates the same. The imaging system is built up of an image-formation optical system comprises, in order from its object side, an aperture stop S, a first positive single lens L1 wherein the absolute value of the axial radius curvature of its image side-surface is smaller than that of its object side-surface, a second negative single lens L2 wherein the absolute value of the axial radius curvature of its image side-surface is smaller than that of its object side-surface and a third positive single lens L3, three single lenses in all, and an image pickup device located on the image plane I of the image-formation optical system.