At the startup of a fuel cell system comprising a fuel cell that generates power upon receiving a supply of a reactive gas and discharges a reactive offgas, a gas passage in which the reactive gas or the reactive offgas flows, a valve device that is installed on the gas passage, and a control device that controls the opening of the valve device, the downstream pressure of the valve device is first acquired (step S3) and, when the downstream pressure is equal to or less than a predetermined pressurization termination pressure (step S5: 'YES'), the valve device is duty-controlled by means of a predetermined duty ratio (step S7, step S9). As a result, the generation of strange noise and the generation of mechanical component stress that arise from sudden pressurization are suppressed.