a heating structure in the form of a sandwich is composed of first and second external elements , each having a face which is directed toward the other element , with the first external element constituting a heating plate , and the heating structure further including a heating unit interposed between the first and second external elements . the heating unit includes at least one substantially flat resistive heat generating element ; a first layer of thermoplastic resin in which the heat generating element is embedded ; an upper electrical insulating sheet and a lower electrical insulating sheet between which the resistive heat generating element and the first layer of thermoplastic resin are interposed to form a sandwich structure , with the first layer of thermoplastic resin adhering to each of the electrical insulating sheets ; and at least one second layer of thermoplastic resin contacting one of the external elements and adhering the one of the external elements to the heating unit . the heating unit further includes at least one further electrical insulating sheet contacting , and adhering to , the at least one second layer of thermoplastic resin ; and an intermediate layer of adhesive material interposed between , and adhering to , one of the upper and lower electrical insulating sheets and the at least one further electrical insulating sheet .