An energy regeneration device which can regenerate energy of a working fluid discharged from an actuator while controlling a flow rate of the working fluid, and a work machine including the device. The regeneration device includes a boom cylinder, an inertial fluid container, an oil tank, an accumulator, a low-pressure-side opening/closing device, and a high-pressure-side opening/closing device. A calculation unit calculates an opening area of each of the low-pressure-side opening/closing device and the high-pressure-side opening/closing device in accordance with a desired flow rate of a hydraulic fluid discharged from the boom cylinder. A regeneration control unit adjusts an opening area of each of the low-pressure-side opening/closing device and the high-pressure-side opening/closing device, and selects alternately the low-pressure-side opening/closing device and the high-pressure-side opening/closing device as a destination with which the inertial fluid container communicates, to supply a discharged hydraulic fluid to an accumulator.