An excavator comprising flow control valves (175B, 175R, 176B, 176R) respectively at rod sides and bottom sides of a plurality of oil hydraulic cylinders (7, 8, 9), the flow control valves (175B, 175R, 176B, 176R) being configured to control flow rates in accordance with pilot pressures, wherein the plurality of oil hydraulic cylinders comprises a boom cylinder and an arm cylinder, and wherein the flow control valves comprise a first flow control valve at a bottom side of the boom cylinder, a second flow control valve at a rod side of the boom cylinder, a third flow control valve at a bottom side of the arm cylinder, and a fourth flow control valve at a rod side of the arm cylinder, the excavator further comprising: a first oil passage connecting the flow control valves (175B, 175R, 176B, 176R) with an oil hydraulic pump-motor which is able to function as an oil hydraulic motor utilizing hydraulic oil flowing out of the plurality of oil hydraulic cylinders (7, 8, 9) and to function as an oil hydraulic pump; and a second oil passage connecting the flow control valves (175B, 175R, 176B, 176R) with a hydraulic oil tank, wherein each of the flow control valves (175B, 175R, 176B, 176R) is configured to switch between allowing and preventing communication with the first oil passage and between allowing and preventing communication with the second oil passage, and wherein the oil hydraulic pump-motor is mechanically connected with a generator, wherein the bottom-side first and third flow control valves (175B, 176B) are configured to adjust a flow rate of hydraulic oil flowing from the associated oil hydraulic cylinder (7, 8) to the first oil passage when the associated oil hydraulic cylinder (7, 8) implements gravity lowering, and wherein supplying part of oil flowing out from the boom cylinder that is lowering to the oil hydraulic pump for power generation to achieve regeneration and supplying part of the oil flowing out from the boom cylinder that is lowering to lower or lift the arm cylinder to achieve regeneration can be performed at the same time. flow control valves (175B, 175R, 176B, 176R) respectively at rod sides and bottom sides of a plurality of oil hydraulic cylinders (7, 8, 9), the flow control valves (175B, 175R, 176B, 176R) being configured to control flow rates in accordance with pilot pressures, wherein the plurality of oil hydraulic cylinders comprises a boom cylinder and an arm cylinder, and wherein the flow control valves comprise a first flow control valve at a bottom side of the boom cylinder, a second flow control valve at a rod side of the boom cylinder, a third flow control valve at a bottom side of the arm cylinder, and a fourth flow control valve at a rod side of the arm cylinder, the excavator further comprising: a first oil passage connecting the flow control valves (175B, 175R, 176B, 176R) with an oil hydraulic pump-motor which is able to function as an oil hydraulic motor utilizing hydraulic oil flowing out of the plurality of oil hydraulic cylinders (7, 8, 9) and to function as an oil hydraulic pump; and a second oil passage connecting the flow control valves (175B, 175R, 176B, 176R) with a hydraulic oil tank, wherein each of the flow control valves (175B, 175R, 176B, 176R) is configured to switch between allowing and preventing communication with the first oil passage and between allowing and preventing communication with the second oil passage, and wherein the oil hydraulic pump-motor is mechanically connected with a generator, wherein the bottom-side first and third flow control valves (175B, 176B) are configured to adjust a flow rate of hydraulic oil flowing from the associated oil hydraulic cylinder (7, 8) to the first oil passage when the associated oil hydraulic cylinder (7, 8) implements gravity lowering, and wherein supplying part of oil flowing out from the boom cylinder that is lowering to the oil hydraulic pump for power generation to achieve regeneration and supplying part of the oil flowing out from the boom cylinder that is lowering to lower or lift the arm cylinder to achieve regeneration can be performed at the same time. a first oil passage connecting the flow control valves (175B, 175R, 176B, 176R) with an oil hydraulic pump-motor which is able to function as an oil hydraulic motor utilizing hydraulic oil flowing out of the plurality of oil hydraulic cylinders (7, 8, 9) and to function as an oil hydraulic pump; and a second oil passage connecting the flow control valves (175B, 175R, 176B, 176R) with a hydraulic oil tank, wherein each of the flow control valves (175B, 175R, 176B, 176R) is configured to switch between allowing and preventing communication with the first oil passage and between allowing and preventing communication with the second oil passage, and wherein the oil hydraulic pump-motor is mechanically connected with a generator, wherein the bottom-side first and third flow control valves (175B, 176B) are configured to adjust a flow rate of hydraulic oil flowing from the associated oil hydraulic cylinder (7, 8) to the first oil passage when the associated oil hydraulic cylinder (7, 8) implements gravity lowering, and wherein supplying part of oil flowing out from the boom cylinder that is lowering to the oil hydraulic pump for power generation to achieve regeneration and supplying part of the oil flowing out from the boom cylinder that is lowering to lower or lift the arm cylinder to achieve regeneration can be performed at the same time.