A hydraulic circuit for a pipe layer, in which a discharge flow rate of a hydraulic pump is controlled by a negative flow control system, the hydraulic circuit comprising: first and second hydraulic pumps (P1, P2) and a pilot pump, which are configured to be connected to an engine; one or more first control valves (3, 4, 5) installed in a center bypass path of the first hydraulic pump and configured to be shifted to control a flow direction and a flow rate of a hydraulic fluid that is supplied to a left traveling motor and a first work apparatus; one or more second control valves (7, 8) installed in a center bypass path of the second hydraulic pump and configured to be shifted to control a flow direction and a flow rate of a hydraulic fluid that is supplied to a right traveling motor and a second work apparatus; a straight traveling valve installed at the upstream side of the center bypass path of the second hydraulic pump, and configured to be shifted by a pilot signal pressure from the pilot pump to cause the hydraulic fluid discharged from the first hydraulic pump to be distributed and supplied to the control valves (3, 7) for the left and right traveling motors and to cause the hydraulic fluid discharged from the second hydraulic pump to be distributed and supplied to the control valves (4, 5, 8) for the first and second work apparatuses when a combined operation mode for simultaneously driving the work apparatus and a traveling apparatus is selected; characterized in that, a pair of unloading valves (50, 50a) is configured to linearly control the closing of a flow path extending from the center bypass paths (2, 6) of the first and second hydraulic pumps (P1, P2) to a hydraulic tank when the work apparatus or the traveling apparatus is finely manipulated in a pipe-laying operation mode; a pilot valve is configured to be shifted by the pilot signal pressure for shifting the straight traveling valve to cause a signal pressure that corresponds to a manipulation signal of the traveling apparatus to be supplied to the unloading valve to close the flow path extending from the center bypass path of the first hydraulic pump to the hydraulic tank and to cause a signal pressure that corresponds to a manipulation signal of the work apparatus to be supplied to the unloading valve to close the flow path extending from the center bypass path of the second hydraulic pump to the hydraulic tank; and an operation mode switching valve is configured to be shifted in response to an electrical signal applied thereto from the outside when a combined operation mode for simultaneously driving the work apparatus and the traveling apparatus is selected to cause the pilot signal pressure from the pilot pump to be supplied to the straight traveling valve, the pilot valve, and valve spools installed at a downstream side of the center bypass paths (2, 6) of the first and second hydraulic pumps (P1, P2), respectively. first and second hydraulic pumps (P1, P2) and a pilot pump, which are configured to be connected to an engine; one or more first control valves (3, 4, 5) installed in a center bypass path of the first hydraulic pump and configured to be shifted to control a flow direction and a flow rate of a hydraulic fluid that is supplied to a left traveling motor and a first work apparatus; one or more second control valves (7, 8) installed in a center bypass path of the second hydraulic pump and configured to be shifted to control a flow direction and a flow rate of a hydraulic fluid that is supplied to a right traveling motor and a second work apparatus; a straight traveling valve installed at the upstream side of the center bypass path of the second hydraulic pump, and configured to be shifted by a pilot signal pressure from the pilot pump to cause the hydraulic fluid discharged from the first hydraulic pump to be distributed and supplied to the control valves (3, 7) for the left and right traveling motors and to cause the hydraulic fluid discharged from the second hydraulic pump to be distributed and supplied to the control valves (4, 5, 8) for the first and second work apparatuses when a combined operation mode for simultaneously driving the work apparatus and a traveling apparatus is selected; a pair of unloading valves (50, 50a) is configured to linearly control the closing of a flow path extending from the center bypass paths (2, 6) of the first and second hydraulic pumps (P1, P2) to a hydraulic tank when the work apparatus or the traveling apparatus is finely manipulated in a pipe-laying operation mode; a pilot valve is configured to be shifted by the pilot signal pressure for shifting the straight traveling valve to cause a signal pressure that corresponds to a manipulation signal of the traveling apparatus to be supplied to the unloading valve to close the flow path extending from the center bypass path of the first hydraulic pump to the hydraulic tank and to cause a signal pressure that corresponds to a manipulation signal of the work apparatus to be supplied to the unloading valve to close the flow path extending from the center bypass path of the second hydraulic pump to the hydraulic tank; and an operation mode switching valve is configured to be shifted in response to an electrical signal applied thereto from the outside when a combined operation mode for simultaneously driving the work apparatus and the traveling apparatus is selected to cause the pilot signal pressure from the pilot pump to be supplied to the straight traveling valve, the pilot valve, and valve spools installed at a downstream side of the center bypass paths (2, 6) of the first and second hydraulic pumps (P1, P2), respectively.