PURPOSE: To process well water and groundwater so that the processed water can be used for city water and the concentrated water therefrom for irrigation.

CONSTITUTION: Well water is drawn and sent to a reverse osmosis membrane module (A). A device for inspection of water quality is provided in the line between the well and the module (A) to periodically inspect the well water. The processed water filtered in the module (A) is used as the city water, while the concentrated water obtd. from the module (A) is stored in a reservoir 11 or the like and used for irrigation of field or the like.