PURPOSE: To increase the output of methanol per piece of reaction tubes by perpendicularly disposing the plural reaction tubes each constructed by packing granular catalysts into a toric spacing constituted of the inside surface of a central tube and the outside surface of the reaction tube.

CONSTITUTION: An unreacted gas flows through an unreacted gas introducing port 11 and a stand pipe 13 and arrives at an upper header 14 and is introduced into the reaction the tube 6 connected to the header via an upper pig tail 16. The reaction tube 6 is the double tubes consisting of the outside tube 61 and the central tube 62. The catalysts 7 held by a catalyst receiving grade 19 supported by a seat 24 are packed into the toric space thereof. The unreacted gas introduced into the reaction tube 6 causes reaction and reaction progresses during the passage through the catalyst 7 layer. The gas with which the reaction ends flows through a lower pig tail 17 and a lower header 15 connected thereto and is taken out of a discharge port 13 for the reacted gas to the outside of the system.