To enable the continuous mass production of drinking water while coping with the deterioration of a reverse osmosis membrane module, when raw seawater comprising deep ocean water is subjected to desalination treatment.

A seawater desalination apparatus has a main head 1 and a sub-head 2 each having the reverse osmosis membrane module. When the reverse osmosis membrane module of the main head 1 is new, only the main head 1 is operated to pass the raw seawater through the main head 1. First permeated water and the raw seawater are mixed in a mixing device 3 to produce first treated water. The drinking water having a prescribed electric conductivity (about 220 μS/cm) is produced in a storage tank 23 by adding the raw seawater to the first treated water. When the reverse osmosis membrane module of the main head 1 is deteriorated, the first permeated water from the main head 1 is passed through the sub-head 2. Second permeated water from the sub-head 2 and concentrated wastewater discharged from the sub-head 2 are mixed in a second mixing device 6 to produce second treated water. The drinking water having the prescribed electric conductivity (about 220 μS/cm) is produced in the storage tank 23 by adding the raw seawater to the second treated water.