PURPOSE: To improve responsiveness by connecting a bypass passage going round a stepless speed control valve to a capacity changing means for a hydraulic motor, and interveniently equipping a changeover valve becoming open at a time of lowering in delivery pressure of a pump in the by pass passage.

CONSTITUTION: A bypass passage 8 going round a stepless speed control valve 20 is provided between a capacity changing means for a hydraulic motor 1 and a tank 10. This bypass passage 8 is provided with a changeover valve 21 becoming open at a time of lowering in delivery pressure of a pump. Hunting phenomenon, thereby, can be prevented on account of moderate reaction time delay brought by an orifice 18, in usual capacity control. Again, when load becomes lighter than a fixed value, the bypass passage 8 becomes open for being able to raise revolution speed on the hydraulic motor 1 at a stretch, therefore responsibility can be improved.