PURPOSE: To prevent lowering of a supply amount of operation oil to main actuators in a hydraulic system in which a plurality of actuators are driven by one hydraulic pump having a limit discharge oil rate lower than a total maximum required oil rate of the actuators by controlling a flow rate of a flow rate control valve without using a pressure sensor when a discharge pressure is a specified value or lower, while controlling a flow rate to a sub-actuator through it.

CONSTITUTION: Operation oil is supplied to a hydraulic motor 2 from a variable volume pump 1 through actuators 5a to 5d for a hydraulic suspension and a flow rate control valve 8. When a discharge pressure is decreased, a piston 47 having a control valve 19 pressurizes a spool in a closing direction, and supply of the operation oil to the hydraulic motor is regulated.