PURPOSE: To prevent generation of a shock upon starting and inching operation without slowing down the motion of an actuator upon normal operation by a method wherein the maximum speed of a member for changing the displacement volume of a pump is increased in accordance with the increase of the absolute value of a detecting signal.

CONSTITUTION: When the operating lever 14 of the control device 12 is operated suddently from a neutral position to the fully positive side thereof, an operating signal XL is increased to XLmax suddenly as shown in the diagram. When the detecting signal YL of a displacement meter 16, serving as a detector to detect the position of a swash plate 8, is in the range of a small value, the minimum speed signal α 1min is outputted, however, the maximum speed signal α 1max is outputted when the detecting signal YL is arriving at the maximum value or the vicinity thereof. Accordingly, the slanting speed of the swash plate 8 becomes small upon the initiation of operation as shown in the diagram and the acceleration of the actuator upon the initiation of operation is small, therefore, the shock upon starting may be reduced and the slow-down of the motion of the actuator 6 may be prevented since the slanting speed of the swash plate 8 is increased gradually thereafter.