To provide an adhesive capable of constructing a strong bond of a material to be bonded by utilizing an unsaturated polyester resin produced by performing the esterification reaction of a carboxylic acid with a glycol, a polystyrene resin-melting effect of styrene monomer contained in a vinyl ester resin, a cross-linking reaction of an unsaturated polyester resin and the polymerization of the styrene monomer.

This adhesive having both characteristics of a thermosetting resin and thermoplastic resin is provided by melting the polystyrene resin, putting it into the unsaturated polyester resin and vinyl ester resin, and completing the cross-linking and polymerization with the same peroxide compound catalysis, and further in the case that the material to be bonded is a polystyrene, it is possible to make the adhesive layer as a one-pieced material by melting a part of its surface on application so as to make the adhesive exhibiting a strong and short time bonding with high reliability.