PURPOSE: To maintain high measurement accuracy by providing an equipment capable of replacing a liquid within a standard pressure detecting tube with a liquid including no air bubble, in a device for measuring a level of the liquid based on a difference in pressure within a system pressure detecting tube in communication with a vapor drum and falling pipe and within the standard pressure detecting tube.

CONSTITUTION: With respect to a pressure tube 2 containing therein fuel 1, such a cooling recirculation system is structured which comprises a rising pipe 3, vapor drum 4, falling pipe 5, manifold 6, recirculation pump 7, pump outlet/inlet pipe 8, lower hedder 9 and an inlet pipe 10. Further, there are formed a standard pressure detecting tube 16 having a condensation tank 15 in communication with the vapor drum 4, and a vapor drum water gauge 13 and a falling pipe water gauge 14 serving as introduction parts for system pressure detecting tubes 17, 17 which are connected with a vapor drum 12 and the mainifold 6, respectively. Cooling water from a recirculation pump seal water feeding piping 18 is supplied to both the water gauges via drain pipe 19, a switching valve 21 and a supply tube 20 and then to the standard pressure detecting tube. Thus, the cooling water can be replaced, so that a level of the fuel may be measured with high accuracy, even if air bubbles should be generated within the standard pressure detecting tube 16 can to accidents.