NOVELTY - The device has a quality-reducing determining unit that determines that quality reduction occurs which is assigned with changes of reactive state of catalyst of a fuel cell unit (20) and a resistor, determines degree of deterioration, if first difference is greater than or equal to prescribed value of first component, determines that quality reduction occurs which is assigned with changes of supply condition of hydrogen and oxygen in cell unit, and determines degree of deterioration, if second difference is greater than or equal to prescribed value of second component.    USE - Quality-reducing detection device for fuel cell unit of vehicle such as two-wheeled motor vehicle and four wheel vehicle.    ADVANTAGE - The cause of quality reduction as a result of actual use of vehicle equipped with the fuel cell unit is determined precisely, so that the condition for recovery of quality reduction is chosen suitably.    DETAILED DESCRIPTION - The device has a value setting unit to set a value obtained by increasing a current actual output value of a fuel cell unit in a step pattern by a predetermined amount target value. A transient response measuring unit performs an output current control of fuel cell unit, such that output current value of fuel cell unit follows preset value, and measures transient response of output voltage of fuel cell unit. A transient response decomposition unit decomposes transient response of output voltage of fuel cell unit into first component representing relatively quick response, and into second component representing relatively slow response. A time constant calculating unit calculates time constants of components. A difference calculating unit calculates two differences that are difference between corresponding reference time constants and time constants.    DESCRIPTION OF DRAWING(S) - The drawings show a block diagram of the quality-reducing detection device, and a configuration diagram of the direct current (DC)-DC transducer. (Drawing includes non-English language text)    Data processing device (10)    Fuel cell unit (20)    Current sensor (32)    Inverter (40)    Battery (70)