A high pressure water electrolysis system includes a high pressure water electrolysis device, a hydrogen storage device, a high pressure hydrogen pipe, a branch pipe, a non-return valve, a pressure detector, and a controller. The controller includes a threshold storage device, a threshold determination device, and a solenoid valve opening/closing operation device. The threshold storage device is configured to store a first threshold of a pressure value detected by the pressure detector and a second threshold lower than the first threshold. The threshold determination device is configured to determine whether or not the pressure value detected by the pressure detector has reached the second threshold. The solenoid valve opening/closing operation device is configured to open and close a solenoid valve if it is determined that the pressure value detected by the pressure detector has reached the second threshold.