To reliably collect the potential energy of a working part to be moved up and down, and recycle the collected energy while reducing the loss.

The hydraulic control system includes an accumulator 35 for accumulating oil discharged from a head side oil chamber 8a of a boom cylinder 8 during the down-movement of a boom, and an intensifier cylinder 17 for intensifying hydraulic pressure in a rod side oil chamber 17b to output it after the oil accumulated by the accumulator 35 is supplied to a head side oil chamber 17a. During up-movement of the boom, the pressure oil intensified by the intensifier cylinder 17 joins oil discharged from a main pump 9. Recirculation oil paths 41, 42, 40, 39 are provided for recirculating the pressure oil output from the rod side oil chamber 17b of the intensifier cylinder 17 to the head side oil chamber 17a. Thus, pressure loss is reduced when the pressure oil passes from the intensifier cylinder 17 through a confluent valve 49 to join the oil discharged from the main pump 9.