A slewing-type working machine comprises: a hydraulic motor 11 having first and second ports 11a and 11b and driving an upper slewing body to slew it; a hydraulic pump 10; a slewing operating device 12 including an operating member 12a; a control valve 13 controlling the hydraulic motor 11 based on an operation signal of the slewing operating device 12; first and second pipe-lines 14 and 15 connecting the first and second ports 11a and 11b of the hydraulic motor 11 to the control valve; communication switching devices 26 and 27 switchable between communication and cutoff between both pipe-lines 14 and 15 and a tank T; a slewing electric motor 30; an electric storage device 31; and a controller 28. During a slewing operation, the controller 28 brings the communication switching devices 26 and 27 into a communicated state and performs regenerative control by issuing a command on a regeneration amount corresponding to a reduction in back pressure by the communication switching devices 26 and 27 in the communicated state to the slewing electric motor 30.
