PURPOSE: To sample a trace quantity of dissolved gas in high temperature, high pressure water and to perform an accurate measurement, by providing an evacuation, cooling mechanism and a supply mechanism which sample automatically a fixed quantity of high temperature, high pressure test water, release gaseous mixture by evacuating the test water and measure the quantit of the gas.

CONSTITUTION: After an evacuation.cooler 6 is evacuated up to set pressure of a vacuum setting device 9 by using a vacuum pump 10, a fixed quantity of test water is sampled from a test water sampling opening 1 to the cooler 6 through an evacuation meachanism 4. Dissolved gaseous mixture is released by the evacuation of the test water and is made into the gaseous mixture which is removed moisture by cooling with cooling water from a flow passage 14. Then, the gaseous mixture is supplied to a piston cylinder 13 through an automatic three-ways switching valve 11 passing through a flow passage 15 and volume of the gaseous mixture is measured accurately in this place. Thereafter, the gaseous mixture is pushed into a pipe 16 by means of a piston by switching the switching valve 11 and is supplied to a gas analyzer 7 to carry out an accurate analysis.