A plurality of mutually separate electrodes supported by a support frame are pre-molded into an integral electrode assembly in which the electrodes are connected with each other through resin moldings. The electrode assembly thus formed is cut off from the support frame to provide a primary product in which the electrodes are electrically disconnected from each other. The primary product is then molded into a molded unit having the electrodes embedded therein. Preferably, the primary product is placed in a mold and supported at a predetermined location by a plurality of support pins which abut against the resin moldings. The support pins are removed from the molded unit while leaving corresponding through-holes after a molten resin is poured into the mold and cooled. The electrodes embedded in the molded unit are prevented from being exposed to the outside of the molded unit via through-holes.