PURPOSE: To effectively prevent pulsation of pressure oil generated in a main line by varying the length of an auxiliary line according to the intensity of a frequency detected by a frequency detecting means, in a device having an auxiliary line to prevent pulsation of a pump delivery pressure.

CONSTITUTION: A pulsation removing device 8a is provided with a number of revolutions detector 12 to detect the number of revolutions of a hydraulic pump 2, a pressure detector 13 to detect the pressure of a main line 7, and a temperature detector 14 to detect a pressure oil temperature. First and second auxiliary lines 9a and 9b in which on-off valves 10 and 11, respectively, are located are connected to a main line 7. By controlling the on-off valves 10 and 11 by a control device 15 by means of output signals from the detectors 12-14, the length of the auxiliary line is varied and pulsation of pressure oil discharged with the aid of a hydraulic pump 2 is prevented from occurring. A length L1 of the first auxiliary line 9a is set to a value suitable for damping of pulsation in the maximum drive state of an actuator 5 and set to a value higher than lengths L2 and L1 of the second auxiliary line 9b.