A flow
rate control device in a hydraulic
excavator, comprising a
hydraulic pump which is driven rotatively by an engine, a hydraulic
actuator which is driven by a hydraulic oil discharged from the
hydraulic pump, a control valve which controls the supply of the hydraulic oil to the hydraulic
actuator, an operating means which operates the control valve so as to change over the valve from one position to another, a
relief valve disposed in a
discharge oil path extending from the
hydraulic pump to limit the
maximum pressure in the
discharge oil path, an operational condition detecting means for detecting an operational condition of the hydraulic
actuator, a
pump pressure detecting means for detecting the
discharge pressure of the hydraulic oil discharged from the hydraulic pump, a flow rate adjusting means for adjusting the discharge flow rate of the hydraulic oil discharged from the hydraulic pump, and a control means to which are inputted detection signals from both the operational condition detecting means and the
pump pressure detecting means. When a specific operational condition of the hydraulic actuator is detected and when the said
discharge pressure is held at a predetermined relief
cut-off pressure or higher for a predetermined period of time, the control means makes control so that the discharge flow rate of the hydraulic oil discharged from the hydraulic pump is decreased to a relief
cut-off pressure by the flow rate adjusting means. According to this construction, the relief
cut-off control can be made only where required for each of various works of different conditions which the hydraulic
excavator performs, such as excavating work, crushing work and land readjusting work. That is, even when the
discharge pressure is held at a predetermined relief cut-off pressure or higher for a predetermined period of time, the relief cut-off control is made if the hydraulic actuator is in the specific operational condition.