According to the present invention, there is disclosed a process for producing a novel resin which comprises the step of copolymerizing a five-member cyclic compound (component A) having a conjugated double bond and represented by the general formula (A) given below:GENERAL FORMULA (A): ##STR1## wherein R is an alkyl group having from 1 to 3 carbon atoms; m and n are zero or integers not less than 1 and are in the relation of m+n=6; and/or a Diels-Alder addition product thereof (A) with a co-dimer (component B) of said five-member cyclic compound and 1,3-butadiene by heating a mixture of 20 to 90 parts by weight of said component A and 80 to 10 parts by weight of said component B at a temperature of from 200.degree. C. to 350.degree. C. Also disclosed are a process for hydrogenating said novel resin to produce a novel hydrogenated resin and resin compositions containing said hydrogenated resin.