To provide a medical oxygen concentrator which is not affected by pressure fluctuation in the delivery side and is equipped with a constant flow rate valve which can stably supply a quantity of oxygen gas as indicated by a prescription.

A flow rate control valve of this pressure fluctuation type medical oxygen concentrator has two chambers 1a, 1b partitioned by a diaphragm 2. Oxygen gas under a prescribed pressure is supplied through branched passages to both chambers. Oxygen gas is flowed into the chamber 1b in the delivery side via a needle 4 equipped on the tip end side of a microvalve 3. The chamber 1b has a spring member 5 to support the diaphragm as keeping balance between the pressure in the chamber 1b and a prescribed pressure in the chamber 1a and a valve part 6 which can adjust gas flow-out so as to keep balance between the pressure in the chamber 1b and the chamber 1a by extension/contraction of the spring member to keep the differential pressure between before and after the needle constant. When a planetary gear device is connected to the microvalve, the flow rate can be adjusted from totally closing to full opening by the angle of rotation within 360°.