PURPOSE: To obtain a double membrane pump having a smallest possible surface area contacting the product so as to cause least possible corrosion for the transfer of the highly purified product by sealing two product chambers from compression medium chambers with two membranes and supplying a transferring medium to compression medium chambers alternately under control.

CONSTITUTION: A suction valve 2, a pressure valve 3 and a suction connection pipe 4, and coaxial product chambers 6 connected to the suction valve 2 and pressure valve 3 are arranged in a central housing 1. The product chambers 6 are sealed by membranes 7 forming compression medium chambers 8. The membrane 7 has two annular toruses 9, 10 which are displaceable peripherally oppositely and radially. A piston rod 13 screwed in the central region of the membrane 7 is guided by a guide 15 via a bush 14 and sealed by a packing 16. When compressed air is supplied to the compression medium chambers 8 via a compressed air pipe 22, the product in one product chamber 6 is pressed and transferred, on the other hand, product is attracted to the expanded left product chamber.