To provide an oxygen/nitrogen separating device capable of efficiently separating and collecting oxygen and nitrogen for a long period while attaining calmness and compactness even in a remarkably simple structure without needing complicated and many constituents.

A 1st pressure circuit 9 for performing unidirectional control of the flow of compressed air from a pressure chamber 4 to an oxygen filter part 7 and a 2nd pressure circuit 10 for performing unidirectional control of the flow of the nitrogen C remaining in the oxygen filter part 7 to the pressure chamber 4 are provided between a discharge port 8 of the pressure chamber 4, in which a piston 3 is incorporated, and the oxygen filter 7 and oxygen B is separated from the nitrogen C in every 2 processes of the piston 3 from the suction work of a suction valve 5 through a compression work to an exhaust work of an exhaust valve 6.