A fuel cell system which is mounted and used for a portable and small electric device comprises a cell unit (1) comprising one or more fuel cells (14), a fuel tank unit (3) for storing a fuel to be supplied to the cell unit (1), a fuel feed unit (4) for supplying the fuel of the fuel tank unit (3) to the cell unit (1), and an opening (7) for supplying an oxidizer gas to the cell unit (1), in a thin housing (2) having a substantially rectangular parallelepiped shape, wherein the fuel tank unit (3), fuel feed unit (4) and cell unit (1) are located at one direction between two opposite short side faces (83a, 83b) of the housing (2). A large-capacity and high-output small fuel cell system is provided by such structure.