NOVELTY - The system (1) has an index value measurement part (6a) for measuring form change index value reflecting form change grade of an electrode catalyst i.e. platinum, of a fuel cell (2), which is formed on an electrolyte layer. A measurement stop part (6b) stops measurement when the change of the measured form change index value satisfies predetermined conditions, which show time of converging as constant value. A measurement resumption part (6d) restarts measurement by the index value measurement part when components in the fuel cell are changed after measurement stop.    USE - Fuel cell system for a vehicle-mounted electric power generation system of a fuel cell vehicle.    ADVANTAGE - The system stops measurement of the form change index value for certain measurement period when the change of the measured form change index value satisfies predetermined condition and when the components in the fuel cell are changed, so that the measurement period for the form change index value can be shortened, and consumption of energy needed for measurement can be reduced. The system can achieve response when the form change index value is fallen by changing the components, so that measurement by the measurement resumption part can be restarted. The system reduces energy loss due to useless measurement when the form change index value is not different from the constant value before components change. The system measures the form change index value based on cyclic-voltammetry characteristics of output current of the fuel cell, which is detected with changing output voltage of the fuel cell, so that the form change grade of the electrode catalyst is measured with high precision.    DETAILED DESCRIPTION - The fuel cell is selected from one of phosphoric-acid type fuel cell, molten carbonate type fuel cell, solid part element electrolyte type fuel cell, direct methanol type fuel cell and a fuel cell having electrolyte layers. An INDEPENDENT CLAIM is also included for a method for controlling a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a fuel cell system.'(Drawing includes non-English language text)'    Fuel cell system (1)    Fuel cell (2)    Index value measurement part (6a)    Measurement stop part (6b)    Measurement resumption part (6d)