PURPOSE: To improve the responsiveness to an abrupt change in load, by constituting a titled device of a measuring device for the concn. of hydrogen ions provided in a supply line for sea water, a calculator which calculates the quantity relation between sea water and sulfuric acid and a controller which controls a feed pump for sulfuric acid.

CONSTITUTION: The pH value of the sea water in the outlet of a decarbonation column 1 is measured with a pH measuring device 11, and the measuring electrical signal corresponding to said value is inputted to a calculator 13. The flow rate of the sea water is measured with a flowmeter 12 for sea water and is inputted to a calculator 13. A set pH value and the measured pH value are compared and the deviation thereof is calculated in the calculator 13, which then calculates the quantity relation between the sea water and sulfuric acid for eliminating the deviation in the pH values and outputs a command electrical signal to a controller 14. The electric signal thereof is inputted to the controller 14 and an electric control signal is fed to a feed pump 9 for sulfuric acid from the controller 14, and the pump 9 is controlled in such a way that the pH value of the sea water flowing in the 1st line 5 for supplying sea water attains a set value.