A metal is attached on a passage-forming core by electrocasting to provide a primary metal layer. A plurality of grooves are formed on the primary metal layer and are filled with filler of low melting point. A metal is attached on the primary metal layer by electrocasting to provide a secondary metal layer. Openings are formed on the secondary metal layer adjacent to its opposite ends so as to communicate with the grooves. The filler in the grooves is melted to provide a plurality of coolant passages. The openings are filled with manifold-forming cores made of filler with low melting point. A metal is attached on the manifold-forming cores by electrocasting to provide tertiary metal layers. Through holes are formed on the tertiary metal layers. The passage-forming core is dissolved and the manifold-forming cores are melted to provide a gas passage and manifolds.