NOVELTY - Provided is a fuel supplying device for a fuel cell system, by which the fuel can be supplied continuously and thus the electric energy can obtained continuously, a portable fuel cell system can be constituted, and the energy loss can be prevented.    ADVANTAGE - The fuel can be supplied continuously. Hence the electric energy can obtained continuously, a portable fuel cell system can be constituted, and the energy loss can be prevented.    DETAILED DESCRIPTION - The fuel cell system comprises: a fuel cell stack part (110) comprising an electrolyte membrane, and a fuel pole (113) and an air pole (114) disposed on the both sides of the electrolyte membrane to generate the electricity by the electrochemical reaction between the fuel and air; a fuel supplying part (120) for supplying a liquid fuel to the fuel pole (113); an air supplying part (130) for supplying air to the air pole (114); an air humidifying part (140) disposed in the middle of the air supplying part (130) for humidifying the air; and an electric energy output part (160) for supplying the electric energy generated in the fuel cell stack part (110) to the load. The fuel supplying device (120) is characterized by comprising: a first fuel tank (121) and a second fuel tank (122) each containing a liquid fuel; a fuel supplying duct (123) for connecting the first fuel tank (121) and the second fuel tank (122) to the fuel pole (114) in parallel; a first fuel cycling duct and a second fuel cycling duct (124a, 124b) for connecting the fuel pole (113) to each of the tanks (121, 122) for recycling the fuel back to the tanks (121, 122) after the reaction in the fuel cell stack part (110); a first fuel switch valve and a second fuel switch valve (125a, 125b) disposed in the middle of the fuel supplying duct (123) and the fuel cycling ducts (124a, 124b), respectively, for connecting the first and the second fuel tanks (121, 122) to the fuel pole (113) in a switching mode; and a fuel pump disposed in the middle of the fuel supplying duct (123) for pumping the fuel in one of the fuel tanks (121, 122) toward the fuel pole (113).