The present invention provides a fuel cell system that can quickly decrease a current/voltage characteristic. A fuel cell system includes an introduction piping which connects a discharging piping and a supply piping in communication with each other, and which introduces part of oxidant-off gas in the supply piping, and a circulation on-off valve provided in the introduction piping. The fuel cell system decreases the supply amount of air by an air compressor when a fuel cell is activated at a low temperature to decrease the air-stoichiometry of the fuel cell, and opens the circulation on-off valve to introduce the oxidant-off gas in the supply piping through the introduction piping, thereby causing the oxidant-off gas to circulate.