A construction machine according to an exemplary embodiment of the present invention includes: an engine; a boom cylinder which is divided into a head side and a rod side; a regeneration line through which hydraulic fluid discharged from the head side of the boom cylinder moves when the boom descends; a regeneration motor which is connected with the regeneration line and is operated, and assists the engine; a pilot pump which generates a pilot pressure; a pilot line through which pilot hydraulic fluid discharged from the pilot pump moves; a regeneration connection line which connects the pilot line and the regeneration line; an on/off valve installed in the regeneration connection line; and a control device which closes the on/off valve when a pressure of the pilot line drops below a predetermined pressure or the hydraulic fluid discharged from the boom cylinder is supplied to the regeneration motor.