To inhibit temperature drop of exhaust gas emitted from a cylinder head more effectively than former exhaust ports.

In forming the exhaust port 2 in the cylinder head 3, a cast iron exhaust port part 5 forming a double shell structure having a surface layer part 4 of which melting point is raised by decarbonization treatment and having a hollow air heat insulating layer 1 formed by making inner section thereof melt and flow by using melting point difference is cast and embedded during casting the cylinder head 3.