PURPOSE: To improve moisture resistivity and heat efficiency by charging an electrically insulating material which is formed by blending a specified amount of inorganic polymer with magnesia powder around a heater wire, impregnating a heat insulating type insulating sheet made of ceramic fiber with inorganic polymer, and then baking the same to obtain a compact.

CONSTITUTION: An electrically insulating material 2 which is formed by blending magnesia powder with 0.2-10 percents by weight of polytitanocarbosilane inorganic polymer is filled in a heating plate to have a compact used as a heater groove 2a. A heater wire 3 is inserted in this groove 2a, and the electrically insulating material 2 is filled into the coil. Further, a heat insulating type insulating sheet 4 which is formed by impregnating inorganic ceramic fiber with inorganic polymer such as borosiloxane is provided thereon, and it is compressed and then baked at about 500°C. Inorganic polymer excellent in electrical insulation property and magnesia powder is mixed in this manner, so the electric insulating property, especially moisture resistivity is improved and the heat efficiency is also improved by the adoption of the heat insulating material on the back surface.