A fuel cell system, which is mounted and used in a portable and small electric device. It comprises a cell unit comprising one or more fuel cells, a fuel tank unit for storing a fuel to be supplied to the cell unit, a fuel feed unit for supplying the fuel of the fuel tank unit to the cell unit, and an opening for supplying an oxidizer gas to the cell unit, in a thin housing having a substantially rectangular parallelepiped shape. The fuel tank unit, fuel feed unit and cell unit are located in one direction between two opposite ends of the housing.