PURPOSE: To eliminate requirement for additional industrial water as diluting water completely by furnishing all amt. of the necessary diluting water with desalted water formed in the process from salt water.

CONSTITUTION: Desalted water fed to a fresh water producing apparatus 10 constituted essentially of reverse osmotic membrane from a first stage desalter 1 and a second stage desalter 2 through a line 13 and 16 respectively is deprived of major portion of oil in a waste water tank 5, and transferred thereafter to a pretreating apparatus 6. The pretreated desalted water is fed to a reverse osmotic membrane module 7 and production of fresh water is commenced, and produced fresh water is stored in a fresh water tank 8. Salt water concentrated to high concn. is discharged to the outside of the system. The fresh water stored in the tank 8 is added to crude water fed to the first stage and the second stage desalters 1, 2 as diluting water for electric salt production through lines 12, 15. Therefore, necessary diluting water is furnished by converting desalted water separated from crude salt water to fresh water sufficiently, and no additional industrial water is required for operation.