Ice in a fuel cell (1) is thawed by passing a hot gas flow in a fuel gas
passage, oxidizing agent gas passage and cooling liquid passage in the
fuel cell (1), and the fuel cell (1) is started from a low temperature
state. When startup processing begins, a gas flow comprising dry, hot gas
in which there is no water or effectively no water is passed through the
fuel cell (1), and subsequently, a steam amount contained in the gas flow
is increased according to the temperature rise of the fuel cell (1).