PURPOSE: To decrease the change of the output against the change of the temperature and improve durability by molding a panel from a composition made of a thermosetting resin and a conducting material, laminating a heating element provided with a pair of electrodes at both ends on it, and coating the surface with an insulating thermoplastic resin.

CONSTITUTION: Melamine formaldehyde resin of 50wt.%, base resin made of α-cellulose of 24wt.%, acetylene black of 23wt.% having the volume resistance value of 2×10-1Ωcm and the average grain size of 40μm, zinc stearate of 2wt.% serving as a mold releasing agent, and phthalic anhydride of 1wt.% serving as a hardening agent are added, crushed, and mixed, and coarse grains are removed by a 48-mesh sieve to form a molding material. It is heated and hardened by a die of 160°C and a compression molding machine of 180kg/cm2, a tin-plated copper stranded wire 3 is fitted to the portion provided on the side face in the longitudinal direction, a heating element 1 is provided at the portion surrounded by it, and the upper and lower faces are pinched by polyethylene sheets as coating layers 2.