The present invention provides a polyfunctional dimethylnaphthalene formaldehyde resin which is polyfunctional, rich in reactivity and useful as a variety of raw materials to be modified, specifically a dimethylnaphthalene formaldehyde resin obtained by allowing (1) one kind or two or more kinds of a dimethylnaphthalene having one methyl group on each of two benzene rings in a naphthalene ring thereof and selected from the group consisting of 1,5-dimethylnaphthalene, 1,6-dimethylnaphthalene, 2,6-dimethylnaphthalene, 1,7-dimethylnaphthalene, 1,8-dimethylnaphthalene and 2,7-dimethylnaphthalene; and (2) formaldehyde to react with each other in the presence of water and an acid catalyst, the polyfunctional dimethylnaphthalene formaldehyde resin having a mean value of the number of hydrogen atoms substituted by the reaction among the six hydrogen atoms directly bonded on the naphthalene ring in the dimethylnaphthalene of from 1.8 to 3.5.