NOVELTY - The fuel cell system has a determination part (61), which determines whether starting temperature when starting a fuel cell is sub-zero temperature; a current-voltage characteristic monitoring part (62), which monitors the current-voltage characteristic of fuel cell; a cell voltage stability monitoring part (63), which monitors the cell voltage stability by electric power generation of fuel cell; and a determination part (64), which determines whether any one of current voltage characteristic and cell voltage stability is more than the threshold-value.    USE - Fuel cell system for vehicle, e.g. fuel cell electric vehicle.    ADVANTAGE - Electric power generation stability is improved since reactive gas is reliably supplied inside in a short time. Water which remains to the flow path where a reactive gas circulates is discharged effectively when the state quantity of reactive gas increases. Noise and vibration characteristic are improved since electric power generation stability is securable and since the state quantity of reactive gas is adjusted accurately. State quantity of reactive gas is accurately adjusted by monitoring simultaneously both a current-voltage characteristic and cell voltage stability when setting the state quantity of reactive gas. Electric power generation stability of fuel cell and power-saving at the time of starting of fuel cell are improved.    DETAILED DESCRIPTION - When starting temperature is sub-zero temperature, the state quantity of a reactive gas is increased to first state quantity. When at least any one of the current-voltage characteristic and cell-voltage stability exceeds a first threshold-value, the state quantity of reactive gas is set to the second state quantity lowered rather than first state quantity.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic block diagram of the control apparatus. (Drawing includes non-English language text).    Control apparatus (45)    Sub-zero temperature determination part (61)    Current-voltage characteristic monitoring part (62)    Cell voltage stability monitoring part (63)    Determination part (64)