PURPOSE: To realize complete removal of residual oxidant with no consideration on the amount of treating agent to be added by feeding sea water, with a predetermined flow rate, to cells filled with granulated calcium sulfite.

CONSTITUTION: In a method for removing residual oxidant from sea water subjected to pasteurization with chlorine or ozone, hydrated calcium sulfite is baked to produce granulated calcium sulfite which is then filled in cells. The cells are then fed with sea water with flow rate lower than 1.0 l/min per 1g of calcium sulfite. When pasteurized sea water containing oxidant is brought into contact with calcium sulfite particles, reaction of (O) + CaSO3 - CaSO4 takes place instantaneously and the reaction does not take place when the oxidant is removed. Since the calcium sulfite is a scarcely dissoluble sub stance, it does not dissolve spontaneously into the sea water and calcium sulfate to be produced is not harmful for living things. According to the method, mea surement of residual oxidant or regulation of the adding amount of treating agent is not required and the residual oxidant can be decomposed substantially completely.