A fuel cell system and a fuel cell vehicle equipped with the fuel cell system are provided. In a case where the voltage of a battery is outside a voltage range of fuel cells where oxidation-reduction proceeds, an ECU controls a DC/DC converter to be placed in a direct connection state (Vbat≈Vfc), and controls a gas supply unit so as to regulate concentration of oxygen or hydrogen supplied to the fuel cells in accordance with a target power generation electric power determined based on electric power required by a load.