A fuel cell system includes a fuel cell body that includes a middle plate and an electricity generating unit that generates electricity by a reaction of air and fuel. The middle plate includes a plurality of unit sections, a supply passage formed inside the middle plate, a supply opening for supplying the fuel to the supply passage, a plurality of inlet openings formed on the unit sections, a discharge passage formed inside the middle plate, a plurality of outlet openings formed on the unit sections, and a discharge opening for discharging the fuel from the discharge passage. The fuel is supplied to the unit sections through the inlet openings, and the fuel discharged from the unit sections being discharged to the discharge passage through the outlet openings. In one embodiment, an opening area of an inlet opening become smaller as the inlet opening is located farther from the supply opening. In another embodiment, a vertical cross-sectional area of the supply passage becomes smaller as the location of the vertical cross-section becomes farther from the supply opening.