A catalyst layer is formed on the metallic surface of a structure constructing a reactor by diffusing a developable metal into the structure by the pack cementation method and developing the diffused metal. The catalyst layer is metallurgically combined with the structure, so that it has excellent mechanical strength and heat conductivity. Thus, by using the structure having the catalyst layer as a heat exchanger type methanation reactor it is possible to obtain a methanation reactor enabling an extremely enhanced rate of reaction to be obtained with superior operability and at great savings.