A process for the solid phase continuous polymerization of polyester in order to achieve a molecular weight increase, measurable by the intrinsic viscosity IV increase of the polyester, wherein the use of at least a reactor (15) is provided, the reactor (15) being cylindrical, rotary around its own central axis (S), substantially horizontal, slightly inclined so as to produce the polymerization of the polyester granules passing through the reactor by gravity thanks to the inclination and the rotation of the reactor (15), inside the reactor there being produced a purge gas flow having the same or the opposite direction with respect to the flow of the polyester granules.