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 including first and second ports (11a, 11b) and receiving supply of hydraulic fluid from one of the first and second ports (11a, 11b) and discharges the hydraulic fluid from the other one of the first and second ports (11a, 11b), thereby slewing the upper slewing body; a hydraulic pump discharging the hydraulic fluid supplied to the hydraulic motor; a slewing operation device including an operating member to which an operation is applied to input a command for the slewing and outputting an operation signal corresponding to the operation applied to the operating member; a control valve which is operated so as to control supply of hydraulic fluid to the hydraulic motor and discharge of hydraulic fluid from the hydraulic motor, based on the operation signal of the slewing operation device; 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 (25, 26) provided between both of first and second pipe-lines (14, 15) and a tank to be switched among a state of cutting off both of the first and second pipe-lines (14, 15) from the tank, a state of bringing the first pipe-line into communication with the tank while cutting off the second pipe-line from the tank, and a state of bringing the second pipe-line into communication with the tank while cutting off the first pipe-line from the tank; and a switching command section inputting a command signal to the communication switching device (25, 26) to switching the states, the switching command section adapted to cause the communication switching device (25, 26) to bring, when the upper slewing body is slewed by the hydraulic motor, only a pipe-line that is one of the first and second pipe-lines (14, 15) and corresponds to a pipe-line on the discharge side of the hydraulic motor into communication with the tank, while bypassing the control valve. a base carrier; an upper slewing body mounted on the base carrier so as to be capable of being slewed; a hydraulic motor including first and second ports (11a, 11b) and receiving supply of hydraulic fluid from one of the first and second ports (11a, 11b) and discharges the hydraulic fluid from the other one of the first and second ports (11a, 11b), thereby slewing the upper slewing body; a hydraulic pump discharging the hydraulic fluid supplied to the hydraulic motor; a slewing operation device including an operating member to which an operation is applied to input a command for the slewing and outputting an operation signal corresponding to the operation applied to the operating member; a control valve which is operated so as to control supply of hydraulic fluid to the hydraulic motor and discharge of hydraulic fluid from the hydraulic motor, based on the operation signal of the slewing operation device; 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 (25, 26) provided between both of first and second pipe-lines (14, 15) and a tank to be switched among a state of cutting off both of the first and second pipe-lines (14, 15) from the tank, a state of bringing the first pipe-line into communication with the tank while cutting off the second pipe-line from the tank, and a state of bringing the second pipe-line into communication with the tank while cutting off the first pipe-line from the tank; and a switching command section inputting a command signal to the communication switching device (25, 26) to switching the states, the switching command section adapted to cause the communication switching device (25, 26) to bring, when the upper slewing body is slewed by the hydraulic motor, only a pipe-line that is one of the first and second pipe-lines (14, 15) and corresponds to a pipe-line on the discharge side of the hydraulic motor into communication with the tank, while bypassing the control valve.