To produce pure water of high quality by suppressing pH variation of inlet water of an RO membrane separation apparatus, and stably keeping pH conditions suitable to RO treatment, in a pure water producing method wherein acid is added to water to be treated, deaerated, and made to pass through the RO membrane separation apparatuses serially arranged in multiple stages.

Water subjected to deaeration 2 is brought into contact with activated carbon 3, and the water after contacting activated carbon is made to pass through the RO membrane separation apparatuses 4-6 serially arranged in multiple stages. By the present invention, deaerated water is treated with activated carbon to stabilize pH, and feed water of the multi-stage RO treatment is easily adjusted to the optimum pH conditions, to stably produce pure water of high purity.