When boom lowering operation for operating a boom cylinder (6) by an external load (W) and an arm cylinder (26) or a bucket cylinder (27) are interlocked in the air, a first changeover valve for boom (2) is controlled to neutral position and a boom regeneration valve (13) is controlled to open position. Since working oil is regenerated from a line (9) on the working oil return side of the boom cylinder (6) through the boom regeneration valve (13) to a line (5) on the working oil supply side, the working oil supply from a hydraulic source (4) to the boom cylinder (6) is not required and a larger quantity of working oil can be supplied from the hydraulic source (4) through a second changeover valve for arm (24) or a changeover valve for bucket (25) to the arm cylinder (26) or the bucket cylinder (27). Working speed of the arm cylinder (26) or the bucket cylinder (27) can thereby be increased and work efficiency of a hydraulic shovel can be enhanced at the time of interconnecting operation.