A fuel cell system quickly and efficiently preheats a frozen stack. The fuel cell system includes: a reformer which generates a reformate gas by reforming a fuel and is heated by a heat source unit; a stack which generates electricity by electrochemically reacting an oxidizer with hydrogen in the reformate gas; and a fluid flow controller which moves air around the reformer into an area around the stack, and which controls airflow on the basis of a temperature change of the stack, wherein the air is heated by the surface temperature of the reformer.