To solve the problem of hunting phenomenon in which when operation is performed at pressure close to the preset pressure for changing over between the regenerating and regeneration cut, state is frequently changed over between the regenerating and the regeneration cut to cause an arm unstable so that the operation can not be smoothly conducted.

A piston 14 for pressing a sub-spool 5 in the direction of energizing a spring 4 with hydraulic pressure of regenerating fluid is provided on the spring side of the sub-spool 5, and a designated pressure difference is provided between the changing over pressure from the regenerative state to the regeneration cut state and the changing over pressure from the regeneration cut state to the regenerative state. This pressure difference prevents frequent changing over between the regenerative state and the regeneration cut state.