Abstract: Mechanical stepless transmission comprising, at least, a pair of trunco-conical pulleys with flexible transmission element or elements, a power transmission mechanism and one oulet planetary gear. One of the pulleys is connected to the stepless transmission input shaft and the other one is connected to the power transmission mechanism input shaft. The output shaft of the power transmission mechanism operates one of the planetary gear elements (satellites carrier, sun or crown gear). The other two elements of this planetary gear are connected, one of them to the input shaft and the other one to the stepless transmission output shaft, the last one constitutes, preferably, the input shaft of a stepped conventional gearbox. Preferably, the power trsnmission mechanism will consist in an intermediate planetary gear.This invention is specially intended for motor vehicles.