PURPOSE: To prevent a measurement part and its piping from being clogged owing to the corrosion of the material of a pipe base for measuring the density in a can, piping, etc., and the deposition of a dissolved body by providing a densitometer to a tank which receives and stores liquid temporarily and measuring the density, and the measuring the amount of the liquid which is fed to the vaporization can by a feed pump by integrating the flow rate measured by a flowmeter and the feed time.

CONSTITUTION: The densitometer 2 is provided to the receiving tank 1 which receives and stores the liquid temporarily and the concentrating device consisting of the vaporization can 4, a heater 5, a circulation line 6, and a circulation pump 7 is emptied before the liquid begins to be processed. Then the liquid stored in the tank 1 is stirred through a stirring line 13 to uniform density and the density of the liquid in the tank 1 is measured by the densitometer 2. After said operation, the liquid feed pump 8 is actuated and a flow rate control valve 10 is opened to hold the liquid level constant by a liquid gauge 9. The liquid in the device is sent downstream from a vaporization can exit line 14 after the vapor and liquid are separated by the heater 5 and can 4, and the flowmeter 15 measures the flow rate at each time.