To perform the heat exchange between a reformed gas and steam to be heat-balanced so that each of the temperature of the reformed gas in a CO purification part, the temperature of the reformed gas in a shift part and the temperature of steam supplied to a reforming part are in an activation temperature region of respective catalysts.

The catalyst in the reforming part is heated into the activation temperature region by a combustion gas to reform fuel and steam into the reformed gas. The reformed gas is heat-exchanged with gas-liquid mixed steam in a main heat exchanger to be cooled into the activation temperature region of the catalyst in the shift part, the steam is heated into the activation temperature region of the catalyst in the reforming part to be supplied to the reforming part and carbon monoxide in the reformed gas and steam are transferred into gaseous hydrogen and carbon dioxide in the shift part. Carbon monoxide contained in the reformed gas discharged from the shift part reacts with air to be oxidized in the CO purification part. The temperature of the reformed gas flowing in the CO purification part is cooled into the activation temperature region of the catalyst in the CO purification part by a CO purification part heat exchanger.