To provide a hydraulic drive system capable of accumulating a hydraulic pressure in a reservoir in order to reduce the capacity of the reservoir while enabling a vehicle to perform long time travel.

A hydraulic drive system 1 selectively uses a driving force of a motor 3 and a driving force of an engine 2 to drive an oil pump 7. A hydraulic pressure is accumulated in a pressure accumulation reservoir 8 by driving the oil pump 7. A planetary gear mechanism 5 is provided to achieve the accumulation. The driving force of the motor 3 is inputted to a sun gear 22. A ring gear 23 is arranged so as to surround the circumference of the sun gear 22 and is provided rotatably around the same rotation axial line as the sun gear 22. A pinion gear 24 is engaged with the sun gear 22 and the ring gear 23. A carrier 21 is provided rotatably centering on the same rotation axial line as the sun gear 22 to support the pinion gear 24 rotatably. The driving force of the engine 2 is inputted to the carrier 21.