PURPOSE: To sharply reduce the amount of corrosion caused by electrolysis by a method wherein the portion equivalent to the inlet and outlet of a passage is constituted by a metal.

CONSTITUTION: The press-pieces 18 and 19, consisting of the metal having a corrosion-resisting property, are fixed on both ends of the cooling material 16 which has a plurality of the penetrated holes 17 to be used for the passage. Then, curved passages 20AW20C of the passage are provided on the press-pieces 18 and 19. Pertaining to the passage in the dotted frames 18A and 18B, an l/d≥1 relations are given between a passage inside diameter d and a passage length l. Accordingly, as the portion equivalent to the inlet and outlet of the passage is made of a corrosion-resisting metal, the amount of corrosion generated by the electrolysis can be greatly reduced even when an electric current is flown at this section.