PURPOSE:To prevent damage to an operator by providing a pilot circuit for giving the fluid pressure between a throttle mechanism and a fluid pressure cylinder as an operating pressure to reduce the set relief pressure for a relief valve when a boom goes down. CONSTITUTION:When a boom is lowered, a pressure difference resulting from a difference in the pressure receiving area of a piston in a cylinder 10 is produced between the pressure fluid supply path F of the cylinder 10 and a return flow path B on which a throttle mechanism 21 acts. By the pressure difference, a flow path switching valve 23 set on a pilot circuit 22 is operated in a condition that a pilot pressure is given through an automatic operating means 24 consisting of pilot paths 24a and 24b, and the set pressure of a relief valve 20 is lowered by pressure from the return flow path B.
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