PURPOSE: To reduce vibration generated at the sudden stop time of an actuator in an early stage by installing the vibration pressure absorbing means in a cylinder oil room.

CONSTITUTION: When a control valve 8 is suddenly changed over from an operation position C to a neutral position A, for example, high pressure is generated in a head side room 16 by the inertia of a boom 20 and transmitted from a supply/discharge oil passage 32 to a vibration pressure relief valve 52. A spool 60 opens a discharge oil passage 36 by the pressure difference acting in a room 66. The movement of the spool 60 is transmitted to a signal generator 50 through a rod 74 and a signal pressure is generated in a signal pressure generation room 76. This signal pressure acts in the room 84 of a change over valve 56 and the spool 80 is moved and the discharge oil passage 36 is closed. Consequently, the high pressure of the head side room 16 is released to an accumulator 54. Then, when the high pressure is generated in a rod side room 14 after reverse turn of vibration pressure, the spools 60 and 80 return and the pressure oil of the accumulator 54 is discharged to an oil tank T. While, the high pressure generated in the rod side room 14 is released to the accumulator 54 by the same operation and the vibration pressure is absorbed.