A slewing-type working machine comprising: a base body; a slewing motor formed of a variable displacement hydraulic motor that is activated to apply a slewing torque for slewing the slewing body to the slewing body by supply of hydraulic fluid to the hydraulic motor; a capacity control device (50, 60) which controls a capacity of the slewing motor; a hydraulic pump which discharges hydraulic fluid to be supplied to the slewing motor; a slewing control device (12, 13) that is operated, by application of a slewing instruction operation for slewing to the slewing control device, to allow hydraulic fluid to be supplied from the hydraulic pump to the slewing motor to activate the slewing motor; a slewing parking brake switchable between a braking state of applying a stop retention force to the slewing body to retain the slewing body in a stopped state and a brake releasing state of releasing the slewing body to let the slewing body be slewed; a brake switching device which switches the slewing parking brake between the braking state and the brake releasing state; and a brake release instruction unit which inputs a brake release instruction to the brake switching device to make the brake switching device switch the slewing parking brake from the braking state to the brake releasing state after the slewing control device is operated, by application of the slewing instruction operation to the slewing control device, to allow hydraulic fluid to be supplied from the hydraulic pump to the slewing motor device, the working machine being characterized in that it further comprises a capacity limiting unit (46, 69, 76) that limits the capacity of the hydraulic motor which capacity is controlled by the slewing control device, the capacity limiting unit being configured to limit the capacity of the slewing motor controlled by the capacity control device to a predetermined brake-release capacity value or less until a brake release point in time when the slewing parking brake is switched to the brake releasing state in response to the input of the brake release instruction to the brake switching device after the slewing control device is operated, by application of the slewing instruction operation to the slewing control device, to allow hydraulic fluid to be supplied from the hydraulic pump to the slewing motor, and configured to permit the capacity control device to increase the capacity of the slewing motor beyond the brake-release capacity value after the brake release point. a base body; a slewing motor formed of a variable displacement hydraulic motor that is activated to apply a slewing torque for slewing the slewing body to the slewing body by supply of hydraulic fluid to the hydraulic motor; a capacity control device (50, 60) which controls a capacity of the slewing motor; a hydraulic pump which discharges hydraulic fluid to be supplied to the slewing motor; a slewing control device (12, 13) that is operated, by application of a slewing instruction operation for slewing to the slewing control device, to allow hydraulic fluid to be supplied from the hydraulic pump to the slewing motor to activate the slewing motor; a slewing parking brake switchable between a braking state of applying a stop retention force to the slewing body to retain the slewing body in a stopped state and a brake releasing state of releasing the slewing body to let the slewing body be slewed; a brake switching device which switches the slewing parking brake between the braking state and the brake releasing state; and a brake release instruction unit which inputs a brake release instruction to the brake switching device to make the brake switching device switch the slewing parking brake from the braking state to the brake releasing state after the slewing control device is operated, by application of the slewing instruction operation to the slewing control device, to allow hydraulic fluid to be supplied from the hydraulic pump to the slewing motor device, the working machine being characterized in that it further comprises a capacity limiting unit (46, 69, 76) that limits the capacity of the hydraulic motor which capacity is controlled by the slewing control device, the capacity limiting unit being configured to limit the capacity of the slewing motor controlled by the capacity control device to a predetermined brake-release capacity value or less until a brake release point in time when the slewing parking brake is switched to the brake releasing state in response to the input of the brake release instruction to the brake switching device after the slewing control device is operated, by application of the slewing instruction operation to the slewing control device, to allow hydraulic fluid to be supplied from the hydraulic pump to the slewing motor, and configured to permit the capacity control device to increase the capacity of the slewing motor beyond the brake-release capacity value after the brake release point.