NOVELTY - Output of a fuel cell in a fuel cell hybrid vehicle comprising a fuel cell as a main power source and storage mechanism as an auxiliary power source, is controlled by controlling the output of the fuel cell according to the amount of electrical energy of the storage mechanism, and according to a load required by the vehicle. The output of the fuel cell is controlled according to the amount of electrical energy of the storage mechanism by a fuel cell stop mode, a fuel cell constant current mode, or a fuel cell-storage mechanism direct connection mode.    USE - Method of controlling output of fuel cell in fuel cell hybrid vehicle (claimed).    ADVANTAGE - The method improves fuel efficiency of a fuel cell hybrid vehicle by operating the fuel cell at a constant power at a maximum efficiency point.    DETAILED DESCRIPTION - Controlling output of a fuel cell in a fuel cell hybrid vehicle comprising a fuel cell as a main power source and storage mechanism as an auxiliary power source, comprises controlling the output of the fuel cell according to the amount of electrical energy of the storage mechanism, and according to a load required by the vehicle. The vehicle further comprises a power converter (120) between the fuel cell and the storage mechanism that provides a constant current operation of the fuel cell, and a directly connected switch for directly connecting/disconnecting between the fuel cell and the storage mechanism. The output of the fuel cell is controlled according to the amount of electrical energy of the storage mechanism by a fuel cell stop mode in which power generation of the fuel cell is stopped, a fuel cell constant current mode in which the power converter is operated so that the fuel cell is operated at a constant power at a maximum efficiency point, or a fuel cell-storage mechanism direct connection mode in which the fuel cell and the storage mechanism are directly connected by the directly connected switch. If the amount of electrical energy of the storage mechanism is below a reference value for restarting the power generation of the fuel cell and if the elapsed time after the fuel cell stop exceeds a predetermined minimum retention time in the fuel cell stop mode, the fuel cell stop mode is switched to the fuel cell constant current mode in a state where the directly connected switch is turned off. If it is below a reference value for starting the fuel cell-storage mechanism direct connection mode in the fuel cell constant current mode, the fuel cell and the storage mechanism are directly connected by the directly connected switch and the operation of the power converter is stopped such that the fuel cell constant current mode is switched to the fuel cell-storage mechanism direct connection mode. In the fuel cell constant current mode, fuel cell balance-of-plant components are controlled to be operated at a constant power in a state where the power converter is controlled to apply a voltage corresponding to the maximum efficiency point to an output side of the fuel cell such that the fuel cell is operated at a constant power in which an output current of the fuel cell is maintained at a constant level. Air is supercharged to a fuel cell stack for a predetermined period of time before the fuel cell and the storage mechanism are directly connected by the directly connected switch.    DESCRIPTION OF DRAWING(S) - The drawing is a block diagram of a power system of a fuel cell-supercapacitor hybrid vehicle.    Power converter (120)    Voltage sensor (124, 132)    Current sensor (133)