Provided are: a regeneration circuit (53) connected to an oil passage (51) through which returning oil of a boom cylinder (3A) flows during a boom-lowering operation, the regeneration circuit having a hydraulic motor (24) to which a generator (25) is connected; a flow rate adjustment circuit (54) connected to the oil passage (51) and having a control valve (5A); an inverter (26) for controlling the flow rate of the regeneration circuit (53) on the basis of a first set flow rate (Q2) established with each operated amount of an operation device (4A); and the control valve (5A) and a proportional valve (27) for controlling the flow rate of the flow rate adjustment circuit (54) on the basis of a second set flow rate (Q1) established with each operated amount of the operation device (4A). A satisfactory feel of operation can thereby be maintained for the operator.