A slewing-type working machine comprising: a base carrier; an upper slewing body mounted on the base carrier so as to be capable of being slewed; a hydraulic motor which includes first and second ports (11a, 11b) and receives supply of hydraulic fluid through one of the ports (11a, 11b) and discharges the hydraulic fluid through the other one of the ports (11a, 11b), thereby driving the upper slewing body to slew the upper slewing body; a hydraulic pump which discharges the hydraulic fluid to be supplied to the hydraulic motor; a slewing electric motor capable of being rotationally driven by the hydraulic motor to perform a regenerative operation; an electricity storage device which stores regenerative power of the slewing electric motor; a slewing operating device including an operating member to which an operation is applied to input a command for the drive to slew, the slewing operation device being adapted to output an operation signal corresponding to the operation applied to the operating member; a control valve which is operated based on the operation signal of the slewing operating device so as to control supply of hydraulic fluid to the hydraulic motor and control discharge of hydraulic fluid from the hydraulic motor; a first pipe-line connecting the first port of the hydraulic motor to the control valve; a second pipe-line connecting the second port of the hydraulic motor to the control valve; characterized by a communication switching device (26, 27) switchable between a communicated state of bringing a pipe-line (14, 15) on an outlet side of the hydraulic motor of the first and second pipe-lines (14, 15) into communication with a tank or a pipe-line (14, 15) on an inlet side of the hydraulic motor of the first and second pipe-lines (14, 15) while bypassing the control valve and a communication-cutoff state for cutting off the communication; an operation detector (32, 33) which detects the operation applied to the operating member of the slewing operating device; and a controller which controls a regenerative operation of the slewing electric motor and switching of the communication switching device (26, 27) based on the detection signal from the operation detector (32, 33), wherein the controller, during a slewing operation of the upper slewing body, switches the communication switching device (26, 27) to the communicated state and performs regenerative control by issuing a command to the slewing electric motor on a regenerative amount corresponding to a reduction in the back pressure by the communication switching device (26, 27). a base carrier; an upper slewing body mounted on the base carrier so as to be capable of being slewed; a hydraulic motor which includes first and second ports (11a, 11b) and receives supply of hydraulic fluid through one of the ports (11a, 11b) and discharges the hydraulic fluid through the other one of the ports (11a, 11b), thereby driving the upper slewing body to slew the upper slewing body; a hydraulic pump which discharges the hydraulic fluid to be supplied to the hydraulic motor; a slewing electric motor capable of being rotationally driven by the hydraulic motor to perform a regenerative operation; an electricity storage device which stores regenerative power of the slewing electric motor; a slewing operating device including an operating member to which an operation is applied to input a command for the drive to slew, the slewing operation device being adapted to output an operation signal corresponding to the operation applied to the operating member; a control valve which is operated based on the operation signal of the slewing operating device so as to control supply of hydraulic fluid to the hydraulic motor and control discharge of hydraulic fluid from the hydraulic motor; a first pipe-line connecting the first port of the hydraulic motor to the control valve; a second pipe-line connecting the second port of the hydraulic motor to the control valve; characterized by a communication switching device (26, 27) switchable between a communicated state of bringing a pipe-line (14, 15) on an outlet side of the hydraulic motor of the first and second pipe-lines (14, 15) into communication with a tank or a pipe-line (14, 15) on an inlet side of the hydraulic motor of the first and second pipe-lines (14, 15) while bypassing the control valve and a communication-cutoff state for cutting off the communication; an operation detector (32, 33) which detects the operation applied to the operating member of the slewing operating device; and a controller which controls a regenerative operation of the slewing electric motor and switching of the communication switching device (26, 27) based on the detection signal from the operation detector (32, 33), wherein the controller, during a slewing operation of the upper slewing body, switches the communication switching device (26, 27) to the communicated state and performs regenerative control by issuing a command to the slewing electric motor on a regenerative amount corresponding to a reduction in the back pressure by the communication switching device (26, 27).