A speed reducer includes two intermediate lines for a turboprop engine, the reducer including: an input line including an input shaft bearing an input gear wheel, a first intermediate line including a first intermediate shaft bearing a first intermediate gear wheel, a second intermediate line including a second intermediate shaft bearing a second intermediate gear wheel, a spring held by a frame, the spring surrounding a longitudinal portion of the input shaft in order to allow a movement of the input gear wheel towards an equilibrium position corresponding to an equal distribution of the transmission of the power coming from the input shaft to the first intermediate shaft of the first intermediate line and the second intermediate shaft of the second intermediate line.