PURPOSE: To uniformly hold a catalyst bed in a reaction tube even when a reforming catalyst is powdered by horizontally inserting a catalyst supporting plate having many holes each having a diameter not permitting the passage of the catalyst into the plural sites of the catalyst bed.

CONSTITUTION: Plural catalyst supporting plates 16 having many holes 15 each having a diameter smaller than that of the reforming catalyst grain and not permitting the passage of the catalyst even when powdered are horizontally fixed to the side walls of the inner tube 3 and outer tube 4 in the inner and outer catalyst beds 6 and 7 in a reaction tube 1. Since the granular catalyst is held by the plate 16 even when powdered, the pressure drop is not increased when the raw gas to be reformed flows through the outer catalyst bed 7 and then through the inner catalyst bed 6. As the part free of the catalyst bed is eliminated, the skin temp. of the reaction tube is not elevated to excess, and the material of the reaction tube 1 is not deteriorated.