| 摘要 |
PROBLEM TO BE SOLVED: To prevent any careless falling of a vehicle body during the operation of an operation lever after starting an engine. SOLUTION: Discharge pressure oil of a hydraulic pump to be driven by an engine is supplied by switching with a direction switching valve 3 to a rod side oil chamber 41 and a bottom side oil chamber 42 of a boom cylinder 4 which oscillate the boom of a hydraulic shovel. In this case, a relief valve 5 capable of switching a relief set pressure to a high pressure and a low pressure to a rod side hydraulic circuit 15 for connecting the rod side oil chamber 41 to the direction switching valve 3 is provided and also switching control means 7, 9, 11, 12, 13, 16 and 17 are provided, which are capable of switching the relief set pressure to high pressure during engine stop and switching to the low pressure when the oil pressure of high pressure and the rod side hydraulic circuit 15 is lower than the relief set pressure during engine drive.
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