NOVELTY - Fuel cell system includes multiphase voltage conversion device (30) with N-phases (N= greater than or equal to 2) that is connected to fuel cell (40); control signal generation portion (130) that generates control signals to control each phase of multiphase voltage conversion device; and impedance calculation portion (140) that measures current of fuel cell and voltage of fuel cell on cycles corresponding to N predetermined sampling frequencies having phase difference equal to predetermined phase difference, and calculates impedance of fuel cell based on measured current and measured voltage.    USE - Fuel cell system for mobile object. Uses include but are not limited to fuel cell hybrid vehicle, electric vehicle, hybrid vehicle, ships, and airplanes.    ADVANTAGE - A detailed sampling result close in shape to the control waveform can be obtained. Thus, the performance of measuring the impedance of the fuel cell in the fuel cell system provided with multiphase voltage conversion device is improved.    DETAILED DESCRIPTION - Fuel cell system comprises fuel cell (40); multiphase voltage conversion device (30) with N-phases (N= greater than or equal to 2) that is connected to the fuel cell; control signal generation portion (130) that generates control signals to control each phase of multiphase voltage conversion device by superimposing a control waveform for measuring impedance on a voltage indicating an output target voltage of the multiphase voltage conversion device, and sequentially outputs the control signals corresponding to N phases with a predetermined phase difference to the multiphase voltage conversion device; and impedance calculation portion (140) that measures current of the fuel cell and voltage of fuel cell on cycles corresponding to N predetermined sampling frequencies having a phase difference equal to the predetermined phase difference, and calculates impedance of the fuel cell based on the measured current and the measured voltage. An INDEPENDENT CLAIM is included for measurement of impedance of fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing is schematic diagram for explaining the measurement of internal impedance of fuel cell.    Control unit (10)    Multiphase voltage conversion device (30)    Fuel cell (40)    Control signal generation portion (130)    Impedance calculation portion (140)