NOVELTY - The system (10) has a controller (28) e.g. ECU, comprising a threshold storage device (64) for storing first threshold of a pressure value detected by a pressure detector at which determination is made whether abnormality is occurred in a pressure reducing valve (18). A threshold determination device (66) determines whether the pressure value detected by the pressure detector reaches second threshold. A solenoid valve opening/closing operation device (68) opens and closes a solenoid valve (20) if the pressure value detected by the detector reaches the second threshold.    USE - High pressure water electrolysis system for producing high pressure hydrogen provided to a hydrogen tank mounted in a fuel-cell electric vehicle and used for electricity generating reaction of fuel cells.    ADVANTAGE - The system allows the solenoid valve to be opened and closed if the secondary side pressure of the pressure reducing valve reaches the second threshold lower than the first threshold at which determining is made that abnormality is occurred in the pressure reducing valve and recoverable seal failure is occurred in the pressure reducing valve, so that the secondary side pressure of the pressure reducing valve lowers, differential pressure between the primary side pressure and the secondary side pressure of the pressure reducing valve can be secured, pressing force in the pressure reducing valve increases, seal failure can be eliminated, desired seal function can be recovered before abnormality occurs in the pressure reducing valve and system operation can be continued, thus suppressing damage to the pressure reducing valve and the solenoid valve on the secondary side.    DETAILED DESCRIPTION - The pressure detector is a pressure sensor (26). An INDEPENDENT CLAIM is also included for a method for activating a high pressure water electrolysis system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a high pressure water electrolysis system.    High pressure water electrolysis system (10)    Pressure reducing valve (18)    Solenoid valve (20)    Pressure sensor (26)    Controller (28)    Threshold storage device (64)    Threshold determination device (66)    Solenoid valve opening/closing operation device (68)