A thermally-fusible adhesive copolymer, said copolymer having a glass transition point in a range of from 100.degree. C. to 250.degree. C., a hygroscopic degree of not more than 1%, and a dielectric rate of not more than 3, said copolymer having a general formula (1) as follows ##STR31## wherein Ar.sub.1 represents a quadrivalent organic radical, Ar.sub.2 represents a divalent organic radical, R represents a divalent organic radical selected from the group consisting of ##STR32## wherein X represents a trivalent bonding radical selected from the group consisting of ##STR33## wherein m and n are each integers of 0 or more, and the sum of m and n is an integer of 1 or more, and wherein 1 is an integer of 1 or more.