A revolving drive control device which has a revolving platform capable of revolving movement and which performs a control for revolvably driving the revolving platform by combining a hydraulically driven revolving hydraulic motor and an electrically driven revolving electric motor, the revolving drive control device comprising: a hydraulic pump for supplying hydraulic fluid for driving to the revolving hydraulic motor; a hydraulic fluid supply control valve for controlling the supply of hydraulic fluid from the hydraulic pump to the revolving hydraulic motor; a power supply source for supplying power for driving to the revolving electric motor; a power supply control device for controlling the supply of power from the power supply source to the revolving electric motor; a revolving operation device operated in order to control the revolving of the revolving platform; characterized in that a drive load detection device (40a,b) for detecting the drive load of the revolving hydraulic motor; and a controller for controlling the actions of the hydraulic fluid supply control valve and the power supply control device on the basis of the operation of the revolving operation device and the drive load detected by the drive load detection device (40a,b); the controller, in correspondence with the operation of the revolving operation device, performing a control whereby hydraulic fluid is supplied from the hydraulic pump to the revolving hydraulic motor by the hydraulic fluid supply control valve so that the revolving platform is revolvably driven by the revolving hydraulic motor, and power for driving is supplied to the revolving electric motor by the power supply control device so that the revolving platform is revolvably driven by the revolving electric motor; and the controller performing a control whereby when the drive load of the revolving hydraulic motor detected by the drive load detection device (40a,b) is a predetermined high load, the revolving platform is revolvably driven by the revolving hydraulic motor and the revolving electric motor, and when the drive load of the revolving hydraulic motor is a predetermined low load, driving by the revolving hydraulic motor is stopped and the revolving platform is revolvably driven by the revolving electric motor. a hydraulic pump for supplying hydraulic fluid for driving to the revolving hydraulic motor; a hydraulic fluid supply control valve for controlling the supply of hydraulic fluid from the hydraulic pump to the revolving hydraulic motor; a power supply source for supplying power for driving to the revolving electric motor; a power supply control device for controlling the supply of power from the power supply source to the revolving electric motor; a revolving operation device operated in order to control the revolving of the revolving platform; characterized in that a drive load detection device (40a,b) for detecting the drive load of the revolving hydraulic motor; and a controller for controlling the actions of the hydraulic fluid supply control valve and the power supply control device on the basis of the operation of the revolving operation device and the drive load detected by the drive load detection device (40a,b); the controller, in correspondence with the operation of the revolving operation device, performing a control whereby hydraulic fluid is supplied from the hydraulic pump to the revolving hydraulic motor by the hydraulic fluid supply control valve so that the revolving platform is revolvably driven by the revolving hydraulic motor, and power for driving is supplied to the revolving electric motor by the power supply control device so that the revolving platform is revolvably driven by the revolving electric motor; and the controller performing a control whereby when the drive load of the revolving hydraulic motor detected by the drive load detection device (40a,b) is a predetermined high load, the revolving platform is revolvably driven by the revolving hydraulic motor and the revolving electric motor, and when the drive load of the revolving hydraulic motor is a predetermined low load, driving by the revolving hydraulic motor is stopped and the revolving platform is revolvably driven by the revolving electric motor.