When the hydrogen separating membrane is in a low temperature condition, a lean bus operation is carried out in a reformer in order to conduct warm-up while suppressing generation of hydrogen. At the timing t1 where the temperature of the hydrogen separator membrane has reached a temperature at which hydrogen embrittlement does not occur, reforming is initiated. In such a condition, oxygen is supplied to hydrogen which is permeated through the hydrogen separator membrane for burning the hydrogen, so as to further facilitate the warm-up. At the timing t2 where the temperature has reached an operation temperature, the supply of oxygen in a purge side is stopped so as to stop the burning of hydrogen, and an operation mode is shifted to a normal operation.