PURPOSE: To prevent occurrence of cavitation by providing a sub-spool in a spool of a hydraulic change-over valve for controlling a hydraulic cylinder and enlarging the opening of the other side path communicating to an oil tank by the operation of the sub-spool when the oil supply side pressure is increased to predetermined pressure.

CONSTITUTION: A sub-spool 29 is provided in a spool 28 of a hydraulic change- over valve 4' for controlling a bucket cylinder 17. The opening of the rod side path leading to an oil tank 37 is enlarged by the operation of the sub-spool 29 when the bottom side pressure in the bucket cylinder 17 is increased to predetermined pressure. When the bucket cylinder 17 is operatively extended and the bottom side pressure exceeds the predetermined pressure, a portion of return oil from the rod side oil chamber 17b bypassing an oil path 31, path 33 and path 32 returns through a pipe line 40 to the oil tank 37 to enlarge the opening of the rod side path leading to the oil tank 37.