An energy regeneration system for a construction machine, the energy regeneration system comprising: first and second variable displacement hydraulic pumps (11, 12); an arm cylinder having a low-pressure chamber connected to the first hydraulic pump through an arm out supply flow path; an arm out return flow path configured to connect a high-pressure chamber of the arm cylinder to a first hydraulic tank; a boom cylinder having a low-pressure chamber connected to the second hydraulic pump through a boom down supply flow path; a boom down return flow path configured to connect a high-pressure chamber of the boom cylinder to a second hydraulic tank; a confluence and regeneration flow path configured to connect the boom down return flow path and the arm out supply flow path to each other in parallel, and regeneratingly supply some of hydraulic fluid, which is returned to the second hydraulic tank by a boom down operation, to the arm out supply flow path during a combined operation of boom down and arm out; and a regeneration flow path configured to connect the boom down return flow path and the boom down supply flow path to each other in parallel, and regeneratingly supply some of hydraulic fluid, which is returned to the second hydraulic tank by the boom down operation, to the low-pressure chamber of the boom cylinder; characterized by detection means configured to detect the pressure of the arm out supply flow path of the arm cylinder and the pressure of the boom down return flow path of the boom cylinder in order to determine whether or not the hydraulic fluid returned to the second hydraulic tank from the boom cylinder can be regenerated during the combined operation of the boom down and the arm out. first and second variable displacement hydraulic pumps (11, 12); an arm cylinder having a low-pressure chamber connected to the first hydraulic pump through an arm out supply flow path; an arm out return flow path configured to connect a high-pressure chamber of the arm cylinder to a first hydraulic tank; a boom cylinder having a low-pressure chamber connected to the second hydraulic pump through a boom down supply flow path; a boom down return flow path configured to connect a high-pressure chamber of the boom cylinder to a second hydraulic tank; a confluence and regeneration flow path configured to connect the boom down return flow path and the arm out supply flow path to each other in parallel, and regeneratingly supply some of hydraulic fluid, which is returned to the second hydraulic tank by a boom down operation, to the arm out supply flow path during a combined operation of boom down and arm out; and a regeneration flow path configured to connect the boom down return flow path and the boom down supply flow path to each other in parallel, and regeneratingly supply some of hydraulic fluid, which is returned to the second hydraulic tank by the boom down operation, to the low-pressure chamber of the boom cylinder; characterized by detection means configured to detect the pressure of the arm out supply flow path of the arm cylinder and the pressure of the boom down return flow path of the boom cylinder in order to determine whether or not the hydraulic fluid returned to the second hydraulic tank from the boom cylinder can be regenerated during the combined operation of the boom down and the arm out.