A hydraulic drive apparatus is mounted on a hydraulic excavator, , comprising a boom directional control valve (23) installed on a hydraulic shovel and controlling a boom cylinder (6) driven by pressure oil delivered from a main hydraulic pump (21), an arm directional control valve (24) controlling an arm cylinder (7), a boom operating device (25) controllably switching the boom directional control valve (23), and an arm operating device (26) controllably switching the arm directional control valve (24). The apparatus also comprises a communication control means allowing the rod side chamber (6b) of the boom cylinder (6) to communicate with the bottom side chamber (7a) of the arm cylinder (7) when the operation amount of the arm operating device (26) exceeds a specified amount (S) so that, in a combined operation in which the pressure oil is supplied to the bottom side chambers of a first hydraulic cylinder and a second hydraulic cylinder, the pressure oil in the rod side chamber of the first hydraulic cylinder which was disposed off in a tank can be effectively utilized irrespective of whether the bottom pressure of the second hydraulic cylinder is high or low. |