This pressure oil energy regeneration device of a work machine is provided with a regeneration hydraulic motor which is driven by return oil, a first hydraulic pump mechanically linked to the regeneration hydraulic motor, a second hydraulic pump which discharges pressure oil that drives a hydraulic actuator, a merge line which merges the pressure oil discharged by a first hydraulic pump with the pressure oil discharged by the second hydraulic pump, a first regulator which can regulate the flow rate of pressure oil in the first hydraulic pump, and a second regulator which can regulate the discharge flow rate of the second hydraulic pump, wherein a control device is provided with a first calculation unit which calculates the non-merge pump flow rate, which is the pump flow rate when driving the hydraulic actuator with only the second hydraulic pump, and calculates a control command to output to the first regulator such that the flow rate of the pressure oil from the first hydraulic pump is less than or equal to the non-merge pump flow rate, and a second calculation unit which calculates a target pump flow rate by subtracting the flow rate of the pressure oil from the first hydraulic pump from the non-merge pump flow rate, and calculates a control command to output to the second regulator for achieving the target pump flow rate.