A hydraulic circuit for a construction machine having a floating function, comprising: two hydraulic pumps (1, 2); a hydraulic cylinder driven by hydraulic fluids supplied from the hydraulic pumps (1,2); a boom driving control valve installed in a flow path between any one of the hydraulic pumps (1, 2) and the hydraulic cylinder and configured to be shifted to control a start, a stop, and a direction change of the hydraulic cylinder; a boom confluence control valve installed in a flow path between the other of the hydraulic pumps (1,2) and the hydraulic cylinder and configured to be shifted to allow the hydraulic fluids discharged from the hydraulic pumps (1, 2) to join together so as to be supplied to a large chamber of the hydraulic cylinder or to allow hydraulic fluids of the large chamber and a small chamber of the hydraulic cylinder to join together so as to be supplied to a hydraulic tank; a manipulation lever configured to output a manipulation signal corresponding to a manipulation amount; characterized by a first pressure sensor configured to measure a pressure of the hydraulic fluid on the large chamber of the hydraulic cylinder; a second pressure sensor configured to measure a boom-down pilot pressure that is applied to an end of the boom driving control valve; a first electronic proportional control valve installed in a flow path between the manipulation lever and the boom confluence control valve and configured to shift the boom confluence control valve to a floating mode by generating the boom-down pilot pressure in proportion to an electrical signal applied thereto and applying the generated boom-down pilot pressure to the boom confluence control valve; a second electronic proportional control valve installed in a flow path between the manipulation lever and the boom driving control valve and configured to supply the hydraulic fluid of the one of the hydraulic pumps (1, 2) to the small chamber of the hydraulic cylinder by generating the boom-down pilot pressure in proportion to the electrical signal applied thereto and applying the generated boom-down pilot pressure to the boom driving control valve; and a controller configured to receive an input of the pressure values detected by the first and second pressure sensors (8, 9), calculate the electrical signal corresponding to the pressure value detected by the second pressure sensor, and apply the calculated electrical signal to the first and second electronic proportional control valves (12, 13). two hydraulic pumps (1, 2); a hydraulic cylinder driven by hydraulic fluids supplied from the hydraulic pumps (1,2); a boom driving control valve installed in a flow path between any one of the hydraulic pumps (1, 2) and the hydraulic cylinder and configured to be shifted to control a start, a stop, and a direction change of the hydraulic cylinder; a boom confluence control valve installed in a flow path between the other of the hydraulic pumps (1,2) and the hydraulic cylinder and configured to be shifted to allow the hydraulic fluids discharged from the hydraulic pumps (1, 2) to join together so as to be supplied to a large chamber of the hydraulic cylinder or to allow hydraulic fluids of the large chamber and a small chamber of the hydraulic cylinder to join together so as to be supplied to a hydraulic tank; a manipulation lever configured to output a manipulation signal corresponding to a manipulation amount; characterized by a first pressure sensor configured to measure a pressure of the hydraulic fluid on the large chamber of the hydraulic cylinder; a second pressure sensor configured to measure a boom-down pilot pressure that is applied to an end of the boom driving control valve; a first electronic proportional control valve installed in a flow path between the manipulation lever and the boom confluence control valve and configured to shift the boom confluence control valve to a floating mode by generating the boom-down pilot pressure in proportion to an electrical signal applied thereto and applying the generated boom-down pilot pressure to the boom confluence control valve; a second electronic proportional control valve installed in a flow path between the manipulation lever and the boom driving control valve and configured to supply the hydraulic fluid of the one of the hydraulic pumps (1, 2) to the small chamber of the hydraulic cylinder by generating the boom-down pilot pressure in proportion to the electrical signal applied thereto and applying the generated boom-down pilot pressure to the boom driving control valve; and a controller configured to receive an input of the pressure values detected by the first and second pressure sensors (8, 9), calculate the electrical signal corresponding to the pressure value detected by the second pressure sensor, and apply the calculated electrical signal to the first and second electronic proportional control valves (12, 13).