The modes of screw instability in a positive column of a nitrogen gas discharge have been studied. Under a certain condition, the critical magnetic field for them=2 mode has been found to be smaller than that for them=1 mode. In addition, the regular oscillations have been observed at the magnetic field larger than the critical one. These phenomena can not be described by the results of Kadomtsev and Nedospasov which are calculated from the small‐amplitude theory. In this paper a theoretical explanation for these phenomena is proposed based on the conception that only the instability which has the maximum growth rate can grow while other instabilities are masked.