Provided is a slewing work machine. The slewing work machine comprises: a lower travel body; an upper slewing body; a hydraulic motor; a slew electric motor; a hydraulic pump; a slewing operation apparatus; a control valve; a brake valve; a communicating valve which switches between an open location, which allows a motor exit-side tube to bypass the control valve and be directly in communication with either a tank or a motor entry-side tube, and a closed location, which blocks off the communication; and a control device which issues a communication valve switch instruction and a torque instruction for the slew electric motor. The control device: (i) derives, as a target value, from an operation state of the slewing operation apparatus and a slewing state of the upper slewing body, either a pressure which arises with the hydraulic motor when the communicating valve is not present, or a torque which is obtained from this pressure; (ii) derives, as an actual value, a pressure which actually arises with the hydraulic motor, or a torque which is obtained from this pressure; and (iii) outputs the torque instruction on the basis of a value which results when the actual value is subtracted from the target value.