PURPOSE: To improve the thermal resistance of a sealing packing, to make any receiving tests unnecessary and to increase the dimensional accuracy of the packing by integrating an ionic-polymerization-hardened layer with the sealing section of one of an upper and a lower case which is formed by a clad plate consisting of either a metallic plate or metallic layers.

CONSTITUTION: After an upper case 1 formed by a clad plate consisting of a stainless steel, a copper and a nickel layer is placed between an over mold 2 and an under mold 3, the upper case 1 is fixed at a given position by means of a pressing pin 5. Next, a reactive nylon material is molten and mixed prior to being poured into a space 6 through an injection hole 7 while the metallic molds are heated by a heater 8, thereby producing a sealing packing 10 consisting of the case 1 and a packing 9 molded together. After that, a thrusting pin 4 is extracted outside the metallic molds. By the means mentioned above, a coin-type sealing upper case which includes a packing and has stable dimensional accuracy can be formed by a reactive nylon material of low viscosity. Therefore it is not necessary to check the resulting product prior to use.