A composition for a high refractive index, low dispersion resin for a close-contact double layer-type composite diffractive optical element that is highly photocurable and provides a high productivity, and a close-contact double layer-type composite diffractive optical element using such composition are provided. A composition for a high refractive index, low dispersion resin for a close-contact double layer-type composite diffractive optical element includes a thiol compound represented by general formula (1) or an oligomer synthesized by use of a thiol compound represented by general formula (1) (A component); and an ene compound including two or more polymerizable unsaturated bonds (B component). (In the formula, p represents an integer of 2 to 5; Xp and Zp independently represent a hydrogen atom or a mercaptomethyl group; a ratio of sulfur atoms in a molecule is 40 to 80% by mass; and the number of thiol groups is 3 or larger.).