It is an object of the present invention to provide a hydraulic drive system that effectively utilizes a hydraulic fluid accumulated in an accumulator to make it possible to achieve a substantial improvement in terms of efficiency for a work machine as a whole. A hydraulic drive system includes: a first hydraulic pump; a first hydraulic actuator driven by being supplied with a hydraulic fluid from the first hydraulic pump; a first line for supplying the hydraulic fluid from the first hydraulic pump to the first hydraulic actuator; a first pump flow adjustment device installed in the first line, the first pump flow adjustment device being configured to adjust; a first hydraulic accumulator for accumulating the hydraulic fluid delivered from the first hydraulic pump; and a first accumulation flow adjustment device configured to adjust the flow of the hydraulic fluid from the first hydraulic pump to the first hydraulic accumulator. The hydraulic drive system further comprises a first accumulation flow supply device configured to supply the hydraulic fluid accumulated in the first hydraulic accumulator to the first hydraulic actuator via a second line different from the first line.