PURPOSE: To prevent a sea water desalination system from depositing sulfuric barium scale by adding strontium nitrate, containing strontium equivalent to 0.1W1pts.wt. of the volume of saturated strontium of sea water, to sea water.

CONSTITUTION: Strontium nitrate, containing strontium equivalent to 0.1W1pts. wt. of the volume of saturated strontium of sea water, is added to sea water to prevent the sea water from separating sulfuric barium scale. As another process, strontium nitrate containing strontium equivalent to 0.1W1pts.wt. of the volume of saturated strontium in the sea water, and one kind or more of high molecular compound of a group constituted of polymaleic acid, polyacryl acid and polyphosphoric acid, 100ppm or less to the sea water, are added to the sea water to prevent the sea water from depositing sulfuric acid barium scale.