A hydraulic control system for a belt drive continuously-variable speed transmission comprising a continuously-variable speed transmission means including a primary variable-pitch pulley having a fixed pulley flange fixed to an input shaft and a movable pulley flange adapted to be axially slidable on said input shaft, a secondary variable-pitch pulley having a fixed pulley flange fixed to an output shaft and a movable pulley flange adapted to be axially slidable on said output shaft, and an endless belt interconnecting said primary and said secondary variable-pitch pulleys; and a hydraulic control circuit including hydraulic servomechanisms hydraulically interconnected with the respective movable pulley flanges of said primary and said secondary variable-pitch pulleys for pressing the corresponding movable pulley flanges toward the corresponding fixed pulley flanges, a regulator valve for supplying the pressurized working fluid of a controlled pressure to said hydraulic servomechanisms, and a control valve capable of generating a pressure signal to change the pressure of the working fluid. The control valve is incorporated into the output shaft and is adapted to generate a pressure signal corresponding to the axial movement of the movable pulley flange of the secondary variable-pitch pulley and to feed the pressure signal to the regulator valve, whereby a pressure factor corresponding to the speed-change ratio of the belt drive continuously-variable speed transmission means is added to the working fluid flowing the pressurized fluid passages of the hydraulic control circuit.