TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Material: The hydrogen rich gas is produced from hydrocarbon group fuel and water vapor.Preferred Method: The temperature of hydrogen rich gas is adjusted by oxidizing portion(s) of flammable component in hydrogen rich gas.    TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Components: The portion (4) is heat exchange portion, and produces water vapor by exchanging heat between mixture of hydrogen rich gas, water and optionally water vapor. The fuel reformer has oxidation portion (3), oxidizing agent adjustment unit (I) (7), oxidizing agent adjustment unit (II), transformation portions (a,b), oxidizing agent adjustment unit (III), driving load detection unit (30), temperature detection unit (11), load variation estimation unit, carbon monoxide oxidation portion (5) and oxidizing agent adjustment unit (IV). The portion (3) oxidizes the flammable component in heat exchange portion upstream. The unit (7) adjusts the flow amount of oxidizing agent supplied to portion (3). The temperature of hydrogen rich gas is adjusted by adjusting the flow amount of oxidizing agent. The portion (3) has hydrogen rich gas flow path packed with catalyst for oxidizing the flammable component. The unit (II) adjusts the flow amount of oxidizing agent supplied to flow path. The transformation portion (a) transforms carbon monoxide in hydrogen rich gas, and portion (b) oxidizes the flammable component. The unit (III) adjusts the flow amount of oxidizing agent supplied to portion (b). The unit (30) measures or estimates the driving load of fuel reformer, and adjusts the temperature of hydrogen rich gas based on driving load. The unit (11) measures or assumes the temperature of portion (4). The flow amount of oxidizing agent introduced to hydrogen rich gas is adjusted according to temperature and driving load of portion (4). The portion (5) selectively oxidizes carbon monoxide and decreases carbon monoxide concentration in hydrogen rich gas.