A method of driving a fuel cell system, comprising: discharging a reformed gas with incomplete composition generated by a reformer by opening a first valve when a reforming reaction starts in the reformer, delivering a reformed gas with complete composition generated by the reformer to a fuel cell stack by closing the first valve and opening a second valve when the reformed gas with complete composition is discharged from the reformer after the reformed gas with incomplete composition is discharged, and storing the reformed gas with complete composition generated by the reformer in a storing container by opening a third valve, determining an internal pressure of the storing container using a signal transmitted from a pressure sensor, comparing the determined internal pressure with a predetermined pressure, closing the third valve when the determined internal pressure reaches the predetermined pressure, and reducing amount of the fuel flowing into the reformer by controlling operation of a fuel booster, opening a fourth valve such that the reformed gas with complete composition stored in the storing container flows into the fuel cell stack when an operation mode is an instantaneous operation mode, determining an expected warm-up time which is time to complete warm-up of the reformer using the ambient temperature detected by a temperature detector, opening degree of the fourth valve is set to full opening during the expected warm-up time, after the expected warm-up time, setting the opening degree of the fourth valve to be smaller considering the amount of the reformed gas supplied from the reformer to the fuel cell stack, and wherein an internal chamber of the storing container is divided into a plurality of chamber, and the reformed gas of different pressure, that are a first pressure and a second pressure, is stored in each chamber, the reformed gas of the first pressure which is higher than the second pressure stored in a first chamber is supplied from the storing container to the fuel cell stack during the initial warm-up, and the reformed gas of the second pressure stored in a second chamber is supplied from the storing container to the fuel cell stack after the initial warm-up. discharging a reformed gas with incomplete composition generated by a reformer by opening a first valve when a reforming reaction starts in the reformer, delivering a reformed gas with complete composition generated by the reformer to a fuel cell stack by closing the first valve and opening a second valve when the reformed gas with complete composition is discharged from the reformer after the reformed gas with incomplete composition is discharged, and storing the reformed gas with complete composition generated by the reformer in a storing container by opening a third valve, determining an internal pressure of the storing container using a signal transmitted from a pressure sensor, comparing the determined internal pressure with a predetermined pressure, closing the third valve when the determined internal pressure reaches the predetermined pressure, and reducing amount of the fuel flowing into the reformer by controlling operation of a fuel booster, opening a fourth valve such that the reformed gas with complete composition stored in the storing container flows into the fuel cell stack when an operation mode is an instantaneous operation mode, determining an expected warm-up time which is time to complete warm-up of the reformer using the ambient temperature detected by a temperature detector, opening degree of the fourth valve is set to full opening during the expected warm-up time, after the expected warm-up time, setting the opening degree of the fourth valve to be smaller considering the amount of the reformed gas supplied from the reformer to the fuel cell stack, and wherein an internal chamber of the storing container is divided into a plurality of chamber, and the reformed gas of different pressure, that are a first pressure and a second pressure, is stored in each chamber, the reformed gas of the first pressure which is higher than the second pressure stored in a first chamber is supplied from the storing container to the fuel cell stack during the initial warm-up, and the reformed gas of the second pressure stored in a second chamber is supplied from the storing container to the fuel cell stack after the initial warm-up.