To damp vibrations while securing the necessary responsiveness by setting appropriate feedback gains which match a lever control input.

A lever control input R and a revolving speed N are detected, and the difference between the inlet pressure and the outlet pressure of a revolving motor 2 is set by a differential pressure setting part 17 so that the deviation from the detected revolving speed N of a desired revolving speed Nr set by the desired speed setting part 16 of a main controller 14 and corresponding to the lever control input R is eliminated. Flow control valves 6, 6 are controlled so that the set differential pressure is obtained. At the differential pressure setting part 17, feedback gains are made variable in accordance with the lever control input R, and in a region wherein the lever control input R is small, the set differential pressure is increased to secure responsiveness; in a region wherein the lever control input is great, the gains are made smaller to prevent excessive response so as to damp vibrations.