A construction machine, comprising: a front work implement having a boom; a double-acting hydraulic cylinder having a rod-side chamber and a bottom-side chamber, driving the front work implement, and acting in the contracting direction when the boom is subjected to its own weight; a main pump for supplying hydraulic fluid to the hydraulic cylinder; an operating device for operating the front work implement; a directional control valve (4, 4A) adapted, when the operating device is operated for the boom to work in a rising direction, to supply hydraulic fluid discharged from the main pump to the bottom-side chamber of the hydraulic cylinder and to return the hydraulic fluid discharged from the rod-side chamber of the hydraulic cylinder to a tank; a discharge line connecting the bottom-side chamber of the hydraulic cylinder to the tank; a hydraulic pump/motor disposed in the discharge line; a first variable restrictor disposed in a portion of the discharge line between the hydraulic pump/motor and the tank; a recovery circuit for connecting a portion of the discharge line between the hydraulic pump/motor and the variable restrictor to the rod-side chamber of the boom cylinder; and a generator/electric motor connected to the hydraulic pump/motor for integral rotation therewith; characterized in that the construction machine further comprises a control unit configured to control the directional control valve (4, 4A) such that the directional control valve (4, 4A) is switched to a position (b) where the hydraulic fluid discharged from the main pump connects to the tank, to control the generator/electric motor as a generator and to control an opening area of the first variable restrictor such that a certain recovery flow rate is supplied from the recovery circuit to the rod-side chamber of the hydraulic cylinder when the operating device is operated for the boom to work in the descending direction in a state in which the front work implement is in a posture allowing to turn in the descending direction by the own weight of the boom, the control unit further configured to control the directional valve (4, 4A) such that the directional control valve (4, 4A) is switched to a position (b) where the hydraulic fluid discharged from the main pump connects to the tank, to control the generator/electric motor as an electric motor and to control the opening area of the first variable restrictor such that the certain recovery flow rate is supplied from the recovery circuit to the rod-side chamber of the hydraulic cylinder when the operating device is operated for the boom to work in the descending direction and the front work implement presses the ground in a state where the front work implement is in contact with the ground. a front work implement having a boom; a double-acting hydraulic cylinder having a rod-side chamber and a bottom-side chamber, driving the front work implement, and acting in the contracting direction when the boom is subjected to its own weight; a main pump for supplying hydraulic fluid to the hydraulic cylinder; an operating device for operating the front work implement; a directional control valve (4, 4A) adapted, when the operating device is operated for the boom to work in a rising direction, to supply hydraulic fluid discharged from the main pump to the bottom-side chamber of the hydraulic cylinder and to return the hydraulic fluid discharged from the rod-side chamber of the hydraulic cylinder to a tank; a discharge line connecting the bottom-side chamber of the hydraulic cylinder to the tank; a hydraulic pump/motor disposed in the discharge line; a first variable restrictor disposed in a portion of the discharge line between the hydraulic pump/motor and the tank; a recovery circuit for connecting a portion of the discharge line between the hydraulic pump/motor and the variable restrictor to the rod-side chamber of the boom cylinder; and a generator/electric motor connected to the hydraulic pump/motor for integral rotation therewith; characterized in that the construction machine further comprises a control unit configured to control the directional control valve (4, 4A) such that the directional control valve (4, 4A) is switched to a position (b) where the hydraulic fluid discharged from the main pump connects to the tank, to control the generator/electric motor as a generator and to control an opening area of the first variable restrictor such that a certain recovery flow rate is supplied from the recovery circuit to the rod-side chamber of the hydraulic cylinder when the operating device is operated for the boom to work in the descending direction in a state in which the front work implement is in a posture allowing to turn in the descending direction by the own weight of the boom, the control unit further configured to control the directional valve (4, 4A) such that the directional control valve (4, 4A) is switched to a position (b) where the hydraulic fluid discharged from the main pump connects to the tank, to control the generator/electric motor as an electric motor and to control the opening area of the first variable restrictor such that the certain recovery flow rate is supplied from the recovery circuit to the rod-side chamber of the hydraulic cylinder when the operating device is operated for the boom to work in the descending direction and the front work implement presses the ground in a state where the front work implement is in contact with the ground.