A construction machine comprising: an engine; a speed sensor for detecting a speed of the engine; a hydraulic pump of a variable displacement type driven by the engine; a hydraulic actuator; a directional control valve for controlling a flow of hydraulic fluid from the hydraulic pump to the hydraulic actuator based on an operation of an operating member; a pilot pump of a fixed displacement type driven by the engine; a regulator device configured to make a displacement of the hydraulic pump variable with using a delivery pressure of the pilot pump; an unloading valve provided on a line connecting a delivery side of the pilot pump to a tank, the unloading valve being switchable between an open position and a closed position; and a control device including a pump displacement control section configured to control the displacement of the hydraulic pump by controlling the regulator device and an unload control section configured to control the unloading valve to the open position at a time of start-up of the engine, characterized in that the pump displacement control section is configured to: store a low speed preset for the engine, the low speed being smaller than a minimally required idle speed of the engine necessary to obtain a delivery pressure of the hydraulic pump capable of driving the hydraulic actuator and larger than a minimally required cranking speed of the engine necessary to obtain a delivery pressure of the pilot pump capable of driving the regulator device; and make the displacement of the hydraulic pump variable to a minimum displacement by controlling the regulator device when the speed of the engine detected by the speed sensor becomes equal to or less than the low speed while the engine is being driven. an engine; a speed sensor for detecting a speed of the engine; a hydraulic pump of a variable displacement type driven by the engine; a hydraulic actuator; a directional control valve for controlling a flow of hydraulic fluid from the hydraulic pump to the hydraulic actuator based on an operation of an operating member; a pilot pump of a fixed displacement type driven by the engine; a regulator device configured to make a displacement of the hydraulic pump variable with using a delivery pressure of the pilot pump; an unloading valve provided on a line connecting a delivery side of the pilot pump to a tank, the unloading valve being switchable between an open position and a closed position; and a control device including a pump displacement control section configured to control the displacement of the hydraulic pump by controlling the regulator device and an unload control section configured to control the unloading valve to the open position at a time of start-up of the engine, characterized in that the pump displacement control section is configured to: store a low speed preset for the engine, the low speed being smaller than a minimally required idle speed of the engine necessary to obtain a delivery pressure of the hydraulic pump capable of driving the hydraulic actuator and larger than a minimally required cranking speed of the engine necessary to obtain a delivery pressure of the pilot pump capable of driving the regulator device; and make the displacement of the hydraulic pump variable to a minimum displacement by controlling the regulator device when the speed of the engine detected by the speed sensor becomes equal to or less than the low speed while the engine is being driven. store a low speed preset for the engine, the low speed being smaller than a minimally required idle speed of the engine necessary to obtain a delivery pressure of the hydraulic pump capable of driving the hydraulic actuator and larger than a minimally required cranking speed of the engine necessary to obtain a delivery pressure of the pilot pump capable of driving the regulator device; and make the displacement of the hydraulic pump variable to a minimum displacement by controlling the regulator device when the speed of the engine detected by the speed sensor becomes equal to or less than the low speed while the engine is being driven.