Operating method of fuel cell system

According to an embodiment of the present invention, an operating method of a fuel cell includes: an operating method of a fuel cell system, including: operation starting step without supplying fuel generating electricity at a predetermined voltage while stopping the supply of fuel; a voltage reducing step of reducing output voltage to targeted voltage; a current increasing rate measuring step of comparing the increasing rate of current with the increasing rate of targeted current; a fuel supply starting step of supplying fuel to the fuel cell stack; and a current following fuel supplying step of controlling fuel concentration and supplying it to be reached to the targeted current.