A gas mixture containing a fuel, water and air is supplied to one end of a reforming room, and a reformed gas containing hydrogen is discharged from the other end thereof. Two or more such reforming units are connected in series, and the upstream part of each reforming room is filled with a first catalyst which catalyzes a partial oxidation reaction in an oxygen-rich environment, and the downstream part is filled with a second catalyst which performs the reforming reaction. The gas mixture which has been heated in a heating unit passes through a distribution tube and is distributed evenly to the reforming units. The reforming room is composed of a reforming tube in which a reforming catalyst is charged, or two or more such reforming tubes, parallel to each other. After being reformed the high-temperature reformed gas is passed around the reforming tubes, and fed back to a manifold.