Control device for working machine

A control device for a working machine according to the present invention comprises a capacity variable pump (2) for supplying working oil, a controller (16) for controlling the discharge amount of the pump, a control valve (3) for controlling the working oil discharged from the pump, a hydraulic actuator (1) operated by the working oil from the control valve, an operating lever (4) for operating the hydraulic actuator, and a relief valve (14) to be operated when the pressure of the working oil is a relief pressure or more. The controller has a dynamic emulation model of a hydraulic driving device, and is constituted to estimate the pressure oil flow rate passing through the relief valve from the emulation model according to the operation amount of the operating lever, and to control the pump flow rate of the pump so that the relief flow rate estimation value gets close to zero. According to this, relief loss can be precisely reduced without generating a response delay or pressure loss, and the power can be saved.