The device for producing ultrapure water according to the reverse osmosis principle, comprising a reverse osmosis filter which is subdivided by the RO membrane into a primary circuit and into a secondary circuit, and a pump for the primary circuit, and a flow resistance valve required downstream of the RO membrane for pressure build-up in the primary circuit is characterized in that at least one cleaning chamber with decalcifying ability and a discharge valve are located in the primary circuit and that the flow in the primary circuit is adjustable via a valve.