A drive control system of an operating machine, the drive control system comprising: a power storage device storing electric power; an electric motor supplied with the electric power from the power storage device to operate and turn a structure of the operating machine; an electric motor driving device configured to adjust the electric power supplied from the power storage device to the electric motor and drive the electric motor at torque corresponding to the supplied electric power; a liquid-pressure motor supplied with an operating liquid to operate and turn the structure in cooperation with the electric motor; an operating liquid supply device configured to adjust a flow rate and liquid pressure of the operating liquid flowing through the liquid-pressure motor and drive the liquid-pressure motor at torque corresponding to the flow rate and liquid pressure of the supplied operating liquid; an input device by which a target turning speed of the structure is input; and a drive control device configured to determine a target torque for accelerating the structure to the target turning speed and control an operation of the electric motor driving device and an operation of the operating liquid supply device such that a sum of the torque of the electric motor and the torque of the liquid-pressure motor becomes the target torque, characterised in that, the drive control system is configured such that when accelerating the structure to the target turning speed, the drive control device controls the operation of the electric motor driving device such that the electric motor drives at high efficiency torque by which a highest electric power efficiency is obtained at each of rotating speeds of the electric motor before the speed of the structure reaches the target turning speed; and the drive control device is configured to control the operation of the operating liquid supply device such that residual torque obtained by subtracting the high efficiency torque from the target torque is output from the liquid-pressure motor. a power storage device storing electric power; an electric motor supplied with the electric power from the power storage device to operate and turn a structure of the operating machine; an electric motor driving device configured to adjust the electric power supplied from the power storage device to the electric motor and drive the electric motor at torque corresponding to the supplied electric power; a liquid-pressure motor supplied with an operating liquid to operate and turn the structure in cooperation with the electric motor; an operating liquid supply device configured to adjust a flow rate and liquid pressure of the operating liquid flowing through the liquid-pressure motor and drive the liquid-pressure motor at torque corresponding to the flow rate and liquid pressure of the supplied operating liquid; an input device by which a target turning speed of the structure is input; and a drive control device configured to determine a target torque for accelerating the structure to the target turning speed and control an operation of the electric motor driving device and an operation of the operating liquid supply device such that a sum of the torque of the electric motor and the torque of the liquid-pressure motor becomes the target torque, characterised in that, the drive control system is configured such that when accelerating the structure to the target turning speed, the drive control device controls the operation of the electric motor driving device such that the electric motor drives at high efficiency torque by which a highest electric power efficiency is obtained at each of rotating speeds of the electric motor before the speed of the structure reaches the target turning speed; and the drive control device is configured to control the operation of the operating liquid supply device such that residual torque obtained by subtracting the high efficiency torque from the target torque is output from the liquid-pressure motor.