A primary object is to provide a hydraulic drive device for a work machine that is capable of optimum control of pump absorption horsepower according to a change of working condition, thereby reducing fuel consumption, and a secondary object is to provide a hydraulic drive device for a work machine that is based on this primary object and capable of restraining variations in the pump absorption horsepower. When working condition identifying unit (41) determines that, for example, a working condition has changed from working condition (a) to (b), engine control system (40a) changes engine output torque characteristic line from engine output torque characteristic line (ELa) to (ELb), and hydraulic pump control system (40b) changes hydraulic pump absorption torque characteristic line from hydraulic pump absorption torque characteristic line (PLa) to (PLb). At output torque points (Ma), (Mb) at which output of engine (17) reaches a maximum, output torque of engine (17) is matched with absorption torque of hydraulic pump (18).