PURPOSE: To reduce the production cost of ultrapure water by extending the water passing life of a membrane separator used in a primary pure water producing apparatus by arranging a filter having a charged filter material built therein to the front stage of the membrane separator of the primary pure water producing apparatus, and treating the filtered water of the filter by the membrane separator.

CONSTITUTION: Raw water is treated in a pretreatment device 2 to remove the suspended matter in the raw water and treated in a two-bed three-tower type pure water producing device 3 using a vacuum degassing tower as a decarbonation tower to produce pure water which is, in turn, treated in a mixed bed type pure water producing device 4 to produce pure water with relatively high specific resistivity. Subsequently, pure water is treated with a filter 5 having a charged filter material built therein to remove fine particles and the filtered water is treated in a membrane separator 6 using a permeable membrane having a pore size smaller than that of the charged filter material to remove remaining fine particles or TOC. The primary pure water obtained in a primary pure water producing apparatus A is received in a storage tank 7 and treated in a secondary pure water producing apparatus B to obtain ultrapure water.