A fuel cell system includes a stack case and an auxiliary device case. The stack case stores a stack including a power generation cell stack body including a plurality of power generation cells stacked horizontally in a stacking direction, and an insulating plate stacked at an end of the power generation cell stack body in the stacking direction. The auxiliary device case stores a fuel cell auxiliary device. The inside of the stack case and the inside of the auxiliary device case that are adjacent to each other in the stacking direction are separated by a partition wall. The partition wall has ventilation connection ports. The ventilation connection ports connect the inside of the stack case with the inside of the auxiliary device case. The insulating plate provided closer to the partition wall, than the power generation cell stack body, inside the stack case faces the ventilation connection ports.