NOVELTY - The system (10) has a hydrogen storage tank (46) provided to a hydrogen pipe (16) to store hydrogen produced by a high-pressure water electrolysis device (14). A first valve mechanism (44) is arranged downstream of the storage tank to allow the hydrogen to flow only from the storage tank to a filling mechanism (42). A bypass pipe (64) is provided to the hydrogen pipe, and connects the electrolysis device and the filling mechanism. A second valve mechanism is arranged on the bypass pipe to allow the hydrogen to flow only from the water electrolysis device to the filling mechanism.    USE - Hydrogen filling system for a fuel cell vehicle.    ADVANTAGE - The high-pressure water electrolysis device can produce hydrogen at same pressure as maximum filling pressure of the hydrogen fuel tank, and a first hydrogen storage tank and a second hydrogen storage tank can store hydrogen at the same pressure as the maximum filling pressure of the hydrogen fuel tank, thus eliminating necessity to raise pressure of the hydrogen by use of a mechanical compressor and reducing consumption energy as much as possible, and hence suppressing needless energy consumption and enhancing cost saving. The hydrogen fuel tank can be filled with hydrogen from the first hydrogen storage tank or the second hydrogen storage tank and filled with hydrogen from the high-pressure water electrolysis device through the bypass pipe simultaneously, thus enabling the hydrogen fuel tank to be easily and speedily filled with hydrogen, and hence reducing time required to fill up the hydrogen fuel tank in an efficient manner.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for operating a hydrogen filling system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of a hydrogen filling system.    Hydrogen filling system (10)    High-pressure water electrolysis device (14)    Hydrogen pipe (16)    Filling mechanism (42)    First valve mechanism (44)    Hydrogen storage tank (46)    Bypass pipe (64)