A construction machine hydraulic drive system including: an engine; a variable displacement hydraulic pump driven by the engine; a plurality of actuators (3a, 3b, 3c...) driven by hydraulic fluid delivered from the hydraulic pump; a plurality of flow and directional control valves (6a, 6b, 6c...) that control a flow rate of the hydraulic fluid supplied from the hydraulic pump to the actuators; a pump control device including: a torque control section that performs absorption torque maintaining control that prevents absorption torque of the hydraulic pump from exceeding predetermined maximum torque (Tmax) by reducing capacity of the hydraulic pump as delivery pressure of the hydraulic pump increases; and a load sensing control section that performs load sensing control that ensures that the delivery pressure of the hydraulic pump is higher than maximum load pressure of the actuators by target differential pressure; an unloading valve connected to a line to which a delivery fluid from the hydraulic pump is guided, the unloading valve opening when the delivery pressure of the hydraulic pump is higher than the sum of the maximum load pressure and an unloading set pressure, to thereby return the delivery fluid of the hydraulic pump to a tank, thus restricting an increase in the delivery pressure of the hydraulic pump; and a pilot pump driven by the engine, characterized in that the hydraulic drive system comprises: an exhaust gas purifying device that purifies exhaust gases from the engine; an instruction device (43, 44, S100) that directs a start of regeneration of the exhaust gas purifying device; and a changeover control device (17d, 15d, 35, 70, S110) that, when the instruction device does not direct the start of the regeneration of the exhaust gas purifying device, enables the load sensing control of the load sensing control section and, when the instruction device directs the start of the regeneration of the exhaust gas purifying device, disables the load sensing control of the load sensing control section to thereby increase the capacity of the hydraulic pump, and guides predetermined pressure generated based on the delivery fluid of the pilot pump to the unloading valve to thereby increase the unloading set pressure. an engine; a variable displacement hydraulic pump driven by the engine; a plurality of actuators (3a, 3b, 3c...) driven by hydraulic fluid delivered from the hydraulic pump; a plurality of flow and directional control valves (6a, 6b, 6c...) that control a flow rate of the hydraulic fluid supplied from the hydraulic pump to the actuators; a pump control device including: a torque control section that performs absorption torque maintaining control that prevents absorption torque of the hydraulic pump from exceeding predetermined maximum torque (Tmax) by reducing capacity of the hydraulic pump as delivery pressure of the hydraulic pump increases; and a load sensing control section that performs load sensing control that ensures that the delivery pressure of the hydraulic pump is higher than maximum load pressure of the actuators by target differential pressure; an unloading valve connected to a line to which a delivery fluid from the hydraulic pump is guided, the unloading valve opening when the delivery pressure of the hydraulic pump is higher than the sum of the maximum load pressure and an unloading set pressure, to thereby return the delivery fluid of the hydraulic pump to a tank, thus restricting an increase in the delivery pressure of the hydraulic pump; and a pilot pump driven by the engine, characterized in that the hydraulic drive system comprises: an exhaust gas purifying device that purifies exhaust gases from the engine; an instruction device (43, 44, S100) that directs a start of regeneration of the exhaust gas purifying device; and a changeover control device (17d, 15d, 35, 70, S110) that, when the instruction device does not direct the start of the regeneration of the exhaust gas purifying device, enables the load sensing control of the load sensing control section and, when the instruction device directs the start of the regeneration of the exhaust gas purifying device, disables the load sensing control of the load sensing control section to thereby increase the capacity of the hydraulic pump, and guides predetermined pressure generated based on the delivery fluid of the pilot pump to the unloading valve to thereby increase the unloading set pressure. an exhaust gas purifying device that purifies exhaust gases from the engine; an instruction device (43, 44, S100) that directs a start of regeneration of the exhaust gas purifying device; and a changeover control device (17d, 15d, 35, 70, S110) that, when the instruction device does not direct the start of the regeneration of the exhaust gas purifying device, enables the load sensing control of the load sensing control section and, when the instruction device directs the start of the regeneration of the exhaust gas purifying device, disables the load sensing control of the load sensing control section to thereby increase the capacity of the hydraulic pump, and guides predetermined pressure generated based on the delivery fluid of the pilot pump to the unloading valve to thereby increase the unloading set pressure.