A waste water purification system for purifying polluted salt water including oil and the like by coagulating and separating pollutant matter in the polluted salt water can regenerate and reuse a coagulant in the system while scarcely resupplying the coagulant, an acid solution and an alkaline solution. The acid and alkaline solutions are required for disintegrating coagulated flocs from the polluted salt water and regenerating the coagulant from sludge. Alkaline water enriched in sodium hydroxide and an acidic aqueous solution containing hydrochloric acid and the like are generated by electrolyzing the purified salt water. The flocs in the separated sludge are disintegrated by use of the alkaline water, pollutant matter is removed from the disintegrated aqueous solution, and the strong acidic containing the hydrochloric acid is added to the aqueous solution removed of the pollutant matter to have the coagulant. The coagulant can be thus regenerated from the recovered sludge.