A gas−liquid flow in a long vertical pipe is considered. A new mathematical model of the flow generalizing the known drift-flux model is constructed. The gas velocity and the average flow velocity in this model are connected by the Zuber−Findlay relation. The model is obtained by averaging the quasi-periodic slug flow regime. Contrary to the drift-flux model, it is possible to restore the internal structure of the flow and, in particular, to give the criterion of transition to the new regime.