PURPOSE: To reduce the occurrence of uneven temperature distribution at a catalyst oxide layer, by a method wherein the terminals of a plurality of pipes for feed gas, which are located in a radial manner to the manifold at the central part of a gas tank, are closed, and plural downward-extending pores are bored at given intervals in the direction of the axis of a pipe.

CONSTITUTION: A pipe 9 has a terminal thereof closed, and pores 10 are bored so that they respectively extend downward at a given angle with the parallel line of a catalyst oxide layer 4 and are positioned at given intervals in the direction of an axis. A plurality of such pipes are radially mounted to a manifold 11. The pipes 9 are rotated through reaction of the gas injected through the pipe 10, and this enables uniformization of gas amount distribution within a gas tank 3.