A fuel cell system for a fuel cell powered vehicle comprising: a fuel cell stack supplied with fuel gas and oxidant gas to generate electric power; a secondary battery; a load; a power conditioner connected to the fuel cell stack, the secondary battery and the load such that the electric power generated by the fuel cell stack is supplied to the load through the power conditioner, electric power discharged by the secondary battery is supplied to the load through the power conditioner, and regenerative electric power charged into the secondary battery from the load is supplied through the power conditioner; and a controller configured to control operation of the power conditioner; wherein the power conditioner comprises a diode configured to prevent electric current from flowing into the fuel cell stack, and a DC/DC converter configured to increase the voltage level of the electric power discharged from the secondary battery and supplied to the load, and to decrease the voltage level of the regenerative electric power supplied from the load and charged into the secondary battery; and characterized in that the power conditioner further comprises a voltmeter configured to detect the output voltage level of the fuel cell stack; and the controller is configured in such a way that, during a start-up operation of the fuel cell stack, a control mode of the DC/DC converter is set to a voltage control mode to allow the output voltage of the DC/DC converter to be controlled, a target value of the output voltage of the DC/DC converter is then set to a voltage level to be greater than a maximum level of a predicted open voltage level of the fuel cell stack, the output voltage level of the fuel cell stack is then measured (S4; S41) and the target value of the output voltage of the DC/DC converter set to be equal to the output level of the fuel cell stack, the converted voltage level of the DC/DC converter is then detected, the DC/DC converter is then set to a power control mode and the target value of the output power of the DC/DC converter is set to be equal to the converted electric power output of the DC/DC converter, the target value of the output power of the DC/DC converter is then decreased at a certain rate until the target value of the output power of the DC/DC converter reaches a predetermined value, at which point the normal control mode of the DC/DC converter is executed and the start-up operation is terminated. a fuel cell stack supplied with fuel gas and oxidant gas to generate electric power; a secondary battery; a load; a power conditioner connected to the fuel cell stack, the secondary battery and the load such that the electric power generated by the fuel cell stack is supplied to the load through the power conditioner, electric power discharged by the secondary battery is supplied to the load through the power conditioner, and regenerative electric power charged into the secondary battery from the load is supplied through the power conditioner; and a controller configured to control operation of the power conditioner; wherein the power conditioner comprises a diode configured to prevent electric current from flowing into the fuel cell stack, and a DC/DC converter configured to increase the voltage level of the electric power discharged from the secondary battery and supplied to the load, and to decrease the voltage level of the regenerative electric power supplied from the load and charged into the secondary battery; and characterized in that the power conditioner further comprises a voltmeter configured to detect the output voltage level of the fuel cell stack; and the controller is configured in such a way that, during a start-up operation of the fuel cell stack, a control mode of the DC/DC converter is set to a voltage control mode to allow the output voltage of the DC/DC converter to be controlled, a target value of the output voltage of the DC/DC converter is then set to a voltage level to be greater than a maximum level of a predicted open voltage level of the fuel cell stack, the output voltage level of the fuel cell stack is then measured (S4; S41) and the target value of the output voltage of the DC/DC converter set to be equal to the output level of the fuel cell stack, the converted voltage level of the DC/DC converter is then detected, the DC/DC converter is then set to a power control mode and the target value of the output power of the DC/DC converter is set to be equal to the converted electric power output of the DC/DC converter, the target value of the output power of the DC/DC converter is then decreased at a certain rate until the target value of the output power of the DC/DC converter reaches a predetermined value, at which point the normal control mode of the DC/DC converter is executed and the start-up operation is terminated.