A hydraulic working machine including a hydraulic drive system provided with a first hydraulic pump and second hydraulic pump for delivering pressure oil to drive a working element which is a boom movable up and down, a doubleacting hydraulic cylinder operable by pressure oil, which has been delivered from the first hydraulic pump and second hydraulic pump, to drive the boom, a first directional control valve for the boom, said first directional control valve being for controlling a flow of pressure oil to be fed from the first hydraulic pump to a bottom chamber or rod chamber of the doubleacting hydraulic cylinder, a second directional control valve for the boom, said second directional control valve being for controlling a flow of pressure oil to be fed from the second hydraulic pump to the bottom chamber or rod chamber of the doubleacting hydraulic cylinder, a control device for switchingly controlling the first directional control valve for the boom and the second directional control valve for the boom, and a jack-up selector valve to be switched when a pressure in the bottom chamber of the doubleacting hydraulic cylinder has reached a predetermined pressure, characterised in that the hydraulic drive system is further provided with a third directional control valve for the boom to control a flow of pressure oil to be fed to the bottom chamber or rod chamber of the doubleacting hydraulic cylinder, said third directional control valve for the boom being switchable by a manipulation of the control device, and also with a third hydraulic pump for feeding pressure oil to the third directional control valve for the boom; and the jack-up selector valve has: a first select position where, when an operation of lowering the boom is performed by a manipulation of the control device and a pressure in the bottom chamber of the doubleacting hydraulic cylinder is higher than the predetermined pressure, the second directional control valve for the boom and the third directional control valve for the boom are held in neutral positions, respectively, to hold permissible the switching of the first directional control valve for the boom by a manipulation of the control device, and a second select position where, when a jack-up operation is performed by the boom lowering manipulation of the control device and a pressure in the bottom chamber of the doubleacting hydraulic cylinder is not higher than the predetermined pressure, the feeding of pressure oil, which is to be delivered from the second hydraulic pump and third hydraulic pump, to the rod chamber of the doubleacting hydraulic cylinder is held permissible in association with the switching of the second directional control valve for the boom and the third directional control valve for the boom by a manipulation of the control device. a first select position where, when an operation of lowering the boom is performed by a manipulation of the control device and a pressure in the bottom chamber of the doubleacting hydraulic cylinder is higher than the predetermined pressure, the second directional control valve for the boom and the third directional control valve for the boom are held in neutral positions, respectively, to hold permissible the switching of the first directional control valve for the boom by a manipulation of the control device, and a second select position where, when a jack-up operation is performed by the boom lowering manipulation of the control device and a pressure in the bottom chamber of the doubleacting hydraulic cylinder is not higher than the predetermined pressure, the feeding of pressure oil, which is to be delivered from the second hydraulic pump and third hydraulic pump, to the rod chamber of the doubleacting hydraulic cylinder is held permissible in association with the switching of the second directional control valve for the boom and the third directional control valve for the boom by a manipulation of the control device.