PURPOSE: To easily carry out the steam reforming of hydrocarbon with high thermal efficiency by using a reaction tube having a double-walled structure and by packing one of the inner and outer tubes with a catalyst for reforming and the other with a catalyst for burning gaseous fuel.

CONSTITUTION: Each reaction tube 10 for reforming is provided with a double- walled structure composed of an inner tube 3 and an outer tube 4. One of the inside 5 of the tube 3 and the inside 6 of the tube 4 is packed with a catalyst for reforming, and the other with a catalyst having a monolithic structure or the like for burning gaseous fuel. The tube 10 is insulated from heat with a heat insulator 11, and starting material for reforming is introduced into the section packed with the catalyst for reforming from the starting material inlet 13. Gaseous fuel is mixed with air, preheated, introduced into the section packed with the catalyst for burning from the fuel inlet 14, and burned. Heat of the combustion is supplied to the reforming section through the wall. The reformed material is taken out of the gaseous product outlet 15. By this method startup and shutdown are easily conducted.