Water contained in cathode effluent from a cathode (1B) of the fuel cell
power plant is condensed by a condenser (8) and recovered to a water tank
(10). Water in the water tank (10) is supplied from a pump (17) to a
humidifier (4) which humidifies hydrogen-rich gas supplied to an anode
(1A) via a water passage (9B). When the power plant stops operating, a
controller (30) first recovers water in the water passage (9B) to the
water tank (10). Also, the freezing probability of the water passage (9B)
is determined from the temperature detected by an outside air temperature
sensor (31), and a wait time is set according to the freezing
probability. By opening a drain valve (15) and draining residual water in
the water passage (9B) after the wait time has elapsed, freezing of the
water passage (9B) can be prevented with a minimum water drainage amount.