Provided is a fuel cell system including: a fuel cell unit; a fuel cartridge; and a fuel actuator that is disposed between the fuel cartridge and the fuel cell unit, to control the fuel supply from the fuel cartridge to the fuel cell unit. The fuel cell unit comprises: a stack including unit cells oriented in parallel to one another; a phase change layer disposed adjacent to the stack and having a length perpendicular to the parallel orientation of the unit cells; a fuel diffusion layer disposed on the phase change layer.