TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Apparatus: The modification reaction part, shift reaction part, carbon monoxide oxidation reaction part, and 1st to 6th heat recovery parts are provided in a container (30). Noble metals such as platinum, rhodium and/or ruthenium are used as catalyst having activity for carbon monoxide transformation for shift reaction part, especially platinum/zirconia is used as catalyst for the shift reaction part. The modification reaction part, shift reaction part and/or carbon monoxide selective oxidation reaction part consists of catalyst having honey-comb structure containing noble metals such as platinum, rhodium and/or ruthenium as catalyst. The raw material gas is maintained at normal pressure. Heat recovery of 2nd, 4th and 6th heat recovery parts is performed by heat exchange with raw material gas or heat recovery fluid which is water. The reformed gas passes through each heat recovery parts. The 2nd, 4th and 6th heat recovery parts have heat transfer fins provided in reformed gas flow path between both reaction parts and a fluid flow path which adjoined to the reformed gas flow path. The heat transfer fin is provided on fluid flow path side. The heat recovery of 1st, 3rd and 5th heat recovery parts is performed by heat exchange with raw material gas or heat recovery fluid, modification reaction part, shift reaction part and carbon monoxide selective oxidation reaction part. The 1st, 3rd and 5th heat recovery parts receive heat by radiation at fluid side and convection from periphery of reaction parts. The modification reaction part, shift reaction part and carbon monoxide selective oxidation reaction part are divided into several partition parts. The 1st, 3rd and 5th heat recovery parts have transfer fin which is configured between partition parts. The reformer has leak detection part for detecting water leakage, air introduction part for mixing oxygen for carbon monoxide selective oxidation to reformed gas, and source which has auxiliary combustion part or electric heater.