A system controller drives a gas circulator with a purge valve closed to supply hydrogen to a fuel cell stack and then increases anode operating pressure and cathode operating pressure to target operating pressures after a predetermined condition is satisfied. This allows an amount of gas mixture of remaining oxygen and hydrogen to be suppressed compared to the case of increasing the anode operating pressure in a state where oxygen remains in the anode circulation path. Accordingly, it is possible to suppress local generation of electric potential and heat and prevent degradation of the components of the fuel cell stack. Moreover, hydrogen is not discharged during activation, which eliminates the need to include dilution equipment of combustion equipment for treating hydrogen and increases the degree of freedom in the layout of the fuel cell system.