To suppress cost and to achieve general purpose application in leveling a load applied to the engine in a working machine such as a hydraulic shovel.

This control system of a working machine includes a hydraulic pump motor 7 which performs pumping by the power of the engine and supplements the power of the engine by motoring, an accumulator 8 which stores the oil discharged from the hydraulic pump motor 7 during the pumping and supplies a pressure oil to the hydraulic pump motor 7 during the motoring, and a control unit 10 for performing an engine load leveling control to level the load applied to the engine 1 by pumping or motoring the hydraulic pump motor 7 according to a working load.