The present invention relates to a system for detecting the remaining quantity of liquid hydrogen stored in a hydrogen storage device. In this system, the remaining quantity of liquid hydrogen is calculated accurately without being affected by the previous state in a tank. The internal pressure of the tank is detected. A fixed heat quantity is applied into the liquid hydrogen. The pressure in the tank after the application of heat quantity is detected. The volume of phase-transited hydrogen gas is calculated based on the heat quantity applied into the tank. The change amount of pressure in the tank before and after the application of heat quantity is calculated based on the pressure and the pressure. The remaining quantity of liquid hydrogen in the tank is calculated based on the volume of hydrogen gas and the change amount of pressure.