The present invention provides oxygen concentrators which achieve reliable pressure control in the oxygen columns with reduced number of components for pressure control in the oxygen columns, and also easy maintenance and reduced power consumption. PSA, PVSA, or VSA oxygen concentrator consisting of oxygen concentration columns BF1 and BF2, pressure control means for adjusting pressure in the oxygen concentration columns, and oxygen tank T for storing concentrated oxygen, comprises mechanical flow control means SC1 which regulates flow from the oxygen tank to the oxygen concentration columns. Pressures in the oxygen concentration columns BF1 and BF2 are kept within a predetermined range by the function of mechanical flow control means SC1 placed in between the oxygen concentration columns and the oxygen tank.