the second embodiment of the present invention when the dozer blade is moved downwardly or descends in the floating mode the hydraulic fluid from the hydraulic tank 7 is replenished to the large chamber of the dozer cylinder 4 through the pilot check valve 11 installed in the flow path 12 for floating so that the downward movement speed or the descending speed of the dozer blade can be increased in addition when the dozer blade is moved upwardly or ascends in the floating mode the hydraulic fluid discharged from the dozer cylinder 4 is promptly returned to the hydraulic tank 7 via the pilot check valve 11 so that the generation of the back pressure in the flow path along which the hydraulic fluid discharged from the dozer cylinder 4 is returned can be minimized to increase the ascending speed of the dozer blade fig4 is a hydraulic circuit diagram of a construction machine having a floating function in accordance with a third preferred embodiment of the present invention the construction machine having a floating function includes a normal mode in which a dozer blade is moved vertically by the manipulation of a manipulation lever and a floating mode in which the dozer blade is moved vertically along a rugged ground surface by its own weight or an external force during the traveling.