A hydraulic drive system for a hydraulic working machine equipped with a swing structure and a working implement, the hydraulic drive system comprising: a hydraulic pump; a swing motor unit driven by a hydraulic fluid delivered from the hydraulic pump, the motor unit including a hydraulic swing motor for rotating the swing structure; a control valve including a swing-directional control valve, the control valve controlling flow of the hydraulic fluid supplied from the hydraulic pump to the hydraulic swing motor; a first swinging relief valve (44a, 44b), disposed inside the swing motor unit, for limiting a driving pressure or braking pressure of the hydraulic swing motor in order to prevent the pressure from exceeding a first setting pressure; a second swinging relief valve (46a, 46b) for limiting the driving pressure or braking pressure of the hydraulic swing motor in order to prevent the pressure from exceeding a second setting pressure that is lower than the first setting pressure; and a selector (47a, 47b) for selecting either one of a first relief mode and a second relief mode, the first relief mode being a mode in which to leave relief characteristics of the first swinging relief valve (44a, 44b) operative intact, the second relief mode being a mode in which to provide relief characteristics obtained by reducing the first setting pressure of the relief characteristics of the first swinging relief valve (44a, 44b) to the second setting pressure of the second swinging relief valve (46a, 46b), characterized in that the first swinging relief valve (44a, 44b) is a shockless relief valve, and the swing motor unit comprises an internal discharge hydraulic line positioned at a downstream side of the first swinging relief valve (44a, 44b) to cause a discharge fluid to flow therefrom, the internal discharge hydraulic line being connected to a tank, and a make-up check valve (45a, 45b) for replenishing an intake end of the hydraulic swing motor with hydraulic fluid from the internal discharge hydraulic line upon the hydraulic swing motor conducting a pumping action during a slowdown or stop of the swing structure; and the second swinging relief valve (46a, 46b) is connected at its downstream side to the internal discharge hydraulic line such that during hydraulic fluid replenishing from the make-up check valve (45a, 45b), a discharge fluid from the second swinging relief valve (46a, 46b) joins that of the first swinging relief valve (44a, 44b) and both fluids are supplied to the make-up check valve (45a, 45b). a hydraulic pump; a swing motor unit driven by a hydraulic fluid delivered from the hydraulic pump, the motor unit including a hydraulic swing motor for rotating the swing structure; a control valve including a swing-directional control valve, the control valve controlling flow of the hydraulic fluid supplied from the hydraulic pump to the hydraulic swing motor; a first swinging relief valve (44a, 44b), disposed inside the swing motor unit, for limiting a driving pressure or braking pressure of the hydraulic swing motor in order to prevent the pressure from exceeding a first setting pressure; a second swinging relief valve (46a, 46b) for limiting the driving pressure or braking pressure of the hydraulic swing motor in order to prevent the pressure from exceeding a second setting pressure that is lower than the first setting pressure; and a selector (47a, 47b) for selecting either one of a first relief mode and a second relief mode, the first relief mode being a mode in which to leave relief characteristics of the first swinging relief valve (44a, 44b) operative intact, the second relief mode being a mode in which to provide relief characteristics obtained by reducing the first setting pressure of the relief characteristics of the first swinging relief valve (44a, 44b) to the second setting pressure of the second swinging relief valve (46a, 46b), characterized in that the first swinging relief valve (44a, 44b) is a shockless relief valve, and the swing motor unit comprises an internal discharge hydraulic line positioned at a downstream side of the first swinging relief valve (44a, 44b) to cause a discharge fluid to flow therefrom, the internal discharge hydraulic line being connected to a tank, and a make-up check valve (45a, 45b) for replenishing an intake end of the hydraulic swing motor with hydraulic fluid from the internal discharge hydraulic line upon the hydraulic swing motor conducting a pumping action during a slowdown or stop of the swing structure; and the second swinging relief valve (46a, 46b) is connected at its downstream side to the internal discharge hydraulic line such that during hydraulic fluid replenishing from the make-up check valve (45a, 45b), a discharge fluid from the second swinging relief valve (46a, 46b) joins that of the first swinging relief valve (44a, 44b) and both fluids are supplied to the make-up check valve (45a, 45b).