the flow rate control valve 57 is shifted to the neutral position and then the swing motor 56 continues to be rotated an overload occurs in the second path 54 a hydraulic fluid corresponding to the overload formed in the second path 54 passes through the fourth check valve 62 installed in the second flow path 63 thus the high pressure hydraulic fluid introduced into the second flow path 63 between the third and fourth check valves 61 and 62 from the second path 54 is stored in the accumulator 65 installed in the regeneration path 64 in this case a hydraulic fluid insufficient in the port u201c a u201d due to continuous rotation of the swing motor 56 is replenished by being sucked in from the hydraulic tank t2 through the first check valve 58 installed in the first flow path 60 as shown in fig3 and a section 3 of fig2 when a spool of the flow rate control valve 57 is shifted to the right on the drawing sheet in response to the control signal applied from the outside the hydraulic pump 51 is connected to the port u201c b u201d of the swing motor 56 through the second path 54 and the port u201c a u201d of the swing motor 56 is connected to the hydraulic tank t2 through the first path 53 thus the swing motor 56 is rotated by the hydraulic fluid supplied thereto from the hydraulic pump 51 through the second path.