A reformer having a combustion unit including a combustion part to burn air and fuel, a combustion gas distributor to distribute the burned combustion gas, and first and second combustion gas passages to guide the distributed combustion gas into an outlet. A fuel-converting catalytic reaction unit includes two reforming reaction parts preparing a reforming reaction reformate, each having a reforming catalyst to reform feed and water supplied from a feed/water inlet, and a water gas shift reaction part preparing a water gas shift reformate, between the two reforming reaction parts to decrease a concentration of carbon monoxide in the reforming reacting reformate. The two units are structured as six cylindrical pipes to realize optimal heat exchange efficiency, and preferential oxidation reactor is provided between first air fuel preheating passages in the combustion part to decrease the concentration of carbon monoxide in the water gas shift reformate to a predetermined level or lower.