A fuel cell stack (2) of a fuel cell power plant (1) generates electric power using a fuel gas such as hydrogen supplied from a fuel gas supply device (3, 5, 5a), and an oxidant gas such as air supplied from an oxidant gas supply system (4). The fuel cell stack (2), fuel gas supply device (3, 5, 5a) and oxidant gas supply system (4) are housed in a casing (6). The casing (6) is provided with a ventilation fan (7). The concentration increase of the fuel gas in the casing (6) due to leaks is appropriately suppressed by the controller (10) controlling the operation of the fan (7) based on the power generation load of the fuel cell stack (2).