To provide a concentration measuring instrument of a dissolved gas, which precisely measures the concentration of the dissolved gas over a long period, and also to provide a concentration measuring method of the dissolved gas.

A tank 4, a space 21S and the pipes 3, 5, 12 and 13 communicating with the tank 4 are filled with water. Subsequently, on-off valves 2, 6 and 14 are closed and a stopper 27 is retracted while a motor 24 is driven to apply predetermined force to a piston 22. Thus, the tank 4, the space 21S and the pipes 3, 5, 12 and 13 are in a pressure reduced state, and the dissolved gas in the water within them becomes gas bubbles to be separated from the water. The piston 22 is moved downward by the occurrence of the gas bubbles. The moving stroke quantity of the piston is read from the graduation of a graduation plate 26.

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