A slewing-type working machine includes: a hydraulic motor having first and second ports and driving an upper slewing body to slew it; a hydraulic pump; a slewing operating device including an operating member; a control valve controlling the hydraulic motor based on an operation signal of the slewing operating device; first and second pipe-lines connecting the first and second ports of the hydraulic motor to the control valve; communication switching devices switchable between communication and cutoff between both pipe-lines and a tank; a slewing electric motor; an electric storage device; and a controller. During a slewing operation, the controller brings the communication switching devices 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 in the communicated state to the slewing electric motor.