Driving mechanism for watch movement having: a barrel mounted on an arbor so as to be capable of turning around an axis of the arbor when the driving mechanism is wound up. A mainspring unit has a first and second spring coiled up inside the barrel in superimposed fashion and coaxial one relative to the other. The first and second springs are coupled at one of their extremities to the barrel and to the arbor, respectively. The unit has a plate mounted coaxially between the two springs, and mounted rotatively on the axis. The first and second springs are coupled at their other extremity to the centre and to the periphery of the plate, respectively, so that the two springs simultaneously wind up around the axis when the driving mechanism is wound up.