A hydrostatic tower whose lower part communicates with a liquid phase space within a flash vessel, and whose upper part communicates with a gas phase space within the flash vessel is provided. A rising liquid level within the hydrostatic tower is detected by at least one maximum liquid level sensor provided at a position at the same level as a predetermined maximum liquid level within the liquid phase space. A dropping liquid level within the hydrostatic tower is detected by at least one minimum liquid level sensor provided at a position at the same level as a predetermined minimum liquid level within the liquid phase space.