A hydraulic drive system is provided with a generator 8 for outputting electricity corresponding to a rotation speed of an engine 7, an electric motor 11 drivable responsive to the electricity from the generator 8, hydraulic pumps 12a, 12b drivable by the electric motor 11, truck-body elevating cylinders 6 for pivoting a truck body 5 in an up-and-down direction, and a stroke detector 14 for detecting a stroke of the control apparatus of the truck-body elevating cylinders 6, and is also provided with a controller 19 for performing control of engine rotation speed. The controller 19 includes a motor electric-power computing means 19a for determining electric power for the electric motor 11, which corresponds to the stroke detected by the stroke detector 14, and a discrimination means 19b for discriminating whether or not the electricity from the generator 8 has become greater than the thus-determined electric power for the electric motor 1.