Device for producing pure water, where the water to be purified is caused to be membrane distilled using one or more units (1), wherein each of the units comprises a space which on its one side comprises a first disc-shaped wall (4,4′) and on its other side a membrane (3,3′) through which gaseous water can pass but not liquid water, and a second disc-shaped wall (5,10), which walls are disposed on different sides of and at a distance from the membrane, wherein water is led in between the first wall (4,4′) and the membrane (3,3′), and wherein the second wall (5,10) is caused to be colder than the water. The invention is characterized in that the first disc-shaped wall (4,4′) is also a membrane through which gaseous water can pass but not liquid water, in that two adjacent membranes (3, 4; 3′ 4′) are supported by a common frame (6), in that the space between the membranes is provided with an inlet opening (7,7′) for water to be purified and an outlet opening (8,8′), in that the second wall (5,10) is a part of a chamber (9) formed by two parallel walls, in that the chamber is provided with an inlet opening (11,11′) for water which is colder than the water to be purified and provided with an outlet opening (12,12′), and in that chambers (9,9′) are arranged in parallel with the membranes at both sides of the frame (6).