A hydraulic drive system for a construction machine, the system comprising: an engine; a variable displacement hydraulic pump driven by the engine; a plurality of actuators (5a, 5b) driven by pressurized hydraulic fluid discharged from the hydraulic pump; a plurality of flow control valves (42a, 42b) for controlling the flow rate of the pressurized hydraulic fluid supplied from the hydraulic pump to the actuators (5a, 5b); maximum load pressure detecting means (6a, 6b) for detecting a maximum load pressure from among the load pressures of the actuators (5a, 5b) when the actuators (5a, 5b) are in operation and detecting tank pressure when the actuators (5a, 5b) are not in operation, thereby outputting the detected pressure as a signal pressure; load sensing control means for controlling the displacement volume of the hydraulic pump such that the discharge pressure of the hydraulic pump becomes higher than the signal pressure by a target differential pressure; an unloading valve connected to a hydraulic fluid supply line through which the pressurized hydraulic fluid discharged from the hydraulic pump is supplied to the flow control valves (42a, 42b), and operative to open to return the hydraulic fluid discharged from the hydraulic pump to a tank when the discharge pressure of the hydraulic pump is higher than the signal pressure by a pressure set for the unloading valve; and a main relief valve connected to the hydraulic fluid supply line and operative to open to return the hydraulic fluid discharged from the hydraulic pump to the tank when the discharge pressure of the hydraulic pump is higher than a first pressure set as relief pressure, thereby limiting maximum pressure in the hydraulic fluid supply line to the first pressure or below; characterized by relief pressure altering means (60, 23, and 24) for manually switching the relief pressure of the main relief valve between the first pressure and a second pressure for engine start-up that is lower than the first pressure and allows the main relief valve to return the hydraulic fluid discharged from the hydraulic pump to the tank in conjunction with the unloading valve when the actuators (5a, 5b) are not in operation. an engine; a variable displacement hydraulic pump driven by the engine; a plurality of actuators (5a, 5b) driven by pressurized hydraulic fluid discharged from the hydraulic pump; a plurality of flow control valves (42a, 42b) for controlling the flow rate of the pressurized hydraulic fluid supplied from the hydraulic pump to the actuators (5a, 5b); maximum load pressure detecting means (6a, 6b) for detecting a maximum load pressure from among the load pressures of the actuators (5a, 5b) when the actuators (5a, 5b) are in operation and detecting tank pressure when the actuators (5a, 5b) are not in operation, thereby outputting the detected pressure as a signal pressure; load sensing control means for controlling the displacement volume of the hydraulic pump such that the discharge pressure of the hydraulic pump becomes higher than the signal pressure by a target differential pressure; an unloading valve connected to a hydraulic fluid supply line through which the pressurized hydraulic fluid discharged from the hydraulic pump is supplied to the flow control valves (42a, 42b), and operative to open to return the hydraulic fluid discharged from the hydraulic pump to a tank when the discharge pressure of the hydraulic pump is higher than the signal pressure by a pressure set for the unloading valve; and a main relief valve connected to the hydraulic fluid supply line and operative to open to return the hydraulic fluid discharged from the hydraulic pump to the tank when the discharge pressure of the hydraulic pump is higher than a first pressure set as relief pressure, thereby limiting maximum pressure in the hydraulic fluid supply line to the first pressure or below; characterized by relief pressure altering means (60, 23, and 24) for manually switching the relief pressure of the main relief valve between the first pressure and a second pressure for engine start-up that is lower than the first pressure and allows the main relief valve to return the hydraulic fluid discharged from the hydraulic pump to the tank in conjunction with the unloading valve when the actuators (5a, 5b) are not in operation.