A fuel cell system that includes a cell unit comprising a fuel cell, a fuel tank unit for storing a fuel to be supplied to the cell unit, and a fuel feed unit for supplying the fuel from the fuel tank unit to the cell unit in a thin housing having a substantially rectangular parallelepiped shape. The fuel tank unit, the fuel feed unit, and the cell unit are located in a specific order in one direction between two opposite ends of the housing. The fuel tank unit includes a valve, which supplies fuel to the fuel feed unit and opens to supply the fuel to the fuel feed unit only when the fuel tank unit is mounted. The fuel feed unit connects sides of the fuel tank unit and the cell unit that face each other and reduces a pressure of a gaseous fuel supplied from the fuel tank unit.