When terminating power generation by a fuel cell 3 in a fuel cell system 1, an amount of a raw fuel material introduced to a reforming catalyst 2a of a reformer 2 is reduced. Here, before the temperature of the reforming catalyst 2a is lowered to the un-reformed gas generation temperature, an amount of water supplied to the reforming catalyst 2a is controlled to increase the temperature of the reforming catalyst 2a. Thus, upon termination of power generation in the fuel cell 3, no un-reformed gas is generated and the reformed gas is supplied to the fuel cell 3.