TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Reformer: In the fuel reformer, the frequency of an operation of the fuel reformer, the operation time of the fuel reformer, the flow volume of the reformed gas produced by execution of the reformation process, O/C ratio which is ratio of the number-of-moles of the carbon with respect to the oxygen concentration in the reformed gas, the hydrocarbon concentration in the reformed gas, and the number-of-moles of oxygen in the mixed gas, based on at least one of the differential pressure of the upstream pressure of the modification catalyst, and a downstream pressure, a fuel consumption, and the temperature of the modification catalyst, the deposition amount estimation means which estimates the deposition amount of the coke deposited on the modification catalyst and an unreacted component is further provided, after execution of the reformation process, the reproduction means performs the reproduction processing, when the deposition amount of the coke estimated by the deposition amount estimation means and an unreacted component exceeds a predetermined amount. During execution of the reproduction processing, the fuel-supply-control means sets feed rate of a fuel to 0. Moreover, a catalyst temperature detection means to detect the temperature of the modification catalyst is provided, the second air supply control means is based on catalyst temperature detected by the catalyst temperature detection means in execution of the reproduction processing, and the second air supply control is started or stopped. The catalyst temperature detection means is an upstream catalyst temperature detection means for it to be provided in the upstream of the modification catalyst, and to detect upstream catalyst temperature, a downstream catalyst temperature detection means for it to be provided in the downstream of the modification catalyst, and to detect downstream catalyst temperature, during execution of the reformation process, the fuel-supply-control means controls feed rate of a fuel based on upstream catalyst temperature detected by the upstream catalyst temperature detection means, during execution of the reformation process, the first air supply control means controls feed rate of air based on upstream catalyst temperature detected by the upstream catalyst temperature detection means, during execution of the reproduction processing, the second air supply control means controls feed rate of air based on downstream catalyst temperature detected by the downstream catalyst temperature detection means.