An object is to provide a sea water desalinating method that is capable of efficiently producing purified water, such as fresh water, from unpurified water, such as sea water. There is a step of carrying out a wastewater treatment by subjecting inorganic wastewater to precipitation and separation. Thus obtained supernatant water is further filtered e.g. by sand filtration means, microfiltration or ultrafiltation and the permeate is directed to the reverse osmosis unit. The RO concentrate is considered low salt concentration water and is mixed, as diluent water, into sea water to produce mixed water. The mixed water prepared by the mixing is finally subjected to reverse osmosis membrane filtration, thereby generating fresh water.