NOVELTY - The system (1) has a control unit (70) that executes low stoichiometry control to decrease both a supplied amount of oxidant gas to a fuel cell stack (10) and a generated electric current produced from the fuel cell stack to less than during idling power generation within a range greater than zero. The control unit decreases the generated electric current from the fuel cell stack as a lowest cell voltage value of fuel cell stack decreases, so that the lowest cell voltage value of fuel cell stack does not fall below a predetermined threshold, during execution of low stoichiometry control.    USE - Fuel cell system for fuel cell vehicle.    ADVANTAGE - Power generation can be maintained in a low stoichiometry state over a long time period by preventing the lowest cell voltage value from declining to below the cancelation threshold, even in case that the cell state is not favorable. Idling stop state can be maintained over a longer period of time since power generation can be more reliably stabilized by decreasing the discharge current accompanying the cancelation determination voltage value, which corresponds to a difference between the lowest cell voltage value and the cancelation threshold and accompanying the power generation stability of stack worsening until there is no alternative but to cancel the idling stop. Idling stop state in which deterioration of the fuel cell stack is low, is maintained over a long time period, irrespective of the state of the cells since idling stop control is executed in accordance with a predetermined idling stop initiation condition having been satisfied during idling power generation, and the fuel cell stack is set to the state of idling stop.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for controlling a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows the block diagram of a fuel cell system.    Fuel cell system (1)    Fuel cell stack (10)    Cell voltage sensor (11)    Supply device (30)    Control unit (70)