To generate hydrogen-rich gas which is supplied to a fuel cell stack (2) of a fuel cell power plant, a reformer (1) generates reformate gas comprising hydrogen from methanol, and a carbon monoxide oxidizer (3) oxidizes carbon monoxide from the reformate gas by an action of a catalyst incorporated therein. A valve (10) which supplies hot gas, and a valve (18) which supplies air are provided. When the catalyst temperature is lower than a predetermined temperature, hot gas is supplied from the valve (10) to the oxidizer (3). Due to the heat of oxidation of carbon monoxide by the air supplied by the valve (18) and the high temperature of the hot gas, the temperature of the catalyst rises rapidly after start-up of the power plant.