A construction machine, comprising: a single rod hydraulic cylinder having a cap chamber and a rod chamber; a first hydraulic pump that is a bidirectionally tiltable variable displacement pump; a cap side hydraulic line that connects one delivery port of the first hydraulic pump and the cap chamber to each other; a rod side hydraulic line that connects another delivery port of the first hydraulic pump and the rod chamber to each other; a fluid tank; a flushing valve that discharges surplus fluid of one of the cap side hydraulic line and the rod side hydraulic line, that one is on a lower pressure side, to the fluid tank; a second hydraulic pump that is a unidirectionally tiltable variable displacement pump; a rod side selector valve that communicates a delivery port of the second hydraulic pump and the rod chamber with each other or interrupts the communication; an operation lever configured to instruct an action of the hydraulic cylinder; a cap pressure sensor that detects a pressure of the cap chamber; a rod pressure sensor that detects a pressure of the rod chamber; and a controller configured to control the first hydraulic pump, the second hydraulic pump, and the rod side selector valve based on inputs from the operation lever, the cap pressure sensor, and the rod pressure sensor; wherein the controller is configured to, in a case where a contraction action of the hydraulic cylinder is instructed through the operation lever, close the rod side selector valve to disable a rod assist action for supplying hydraulic fluid from the second hydraulic pump to the rod chamber when a differential pressure obtained by subtracting the pressure of the cap chamber from the pressure of the rod chamber is equal to or lower than a first threshold value set to a level equal to or higher than a selection setting pressure of the flushing valve, and open the rod side selector valve to enable the rod assist action when the differential pressure is higher than the first threshold value. a single rod hydraulic cylinder having a cap chamber and a rod chamber; a first hydraulic pump that is a bidirectionally tiltable variable displacement pump; a cap side hydraulic line that connects one delivery port of the first hydraulic pump and the cap chamber to each other; a rod side hydraulic line that connects another delivery port of the first hydraulic pump and the rod chamber to each other; a fluid tank; a flushing valve that discharges surplus fluid of one of the cap side hydraulic line and the rod side hydraulic line, that one is on a lower pressure side, to the fluid tank; a second hydraulic pump that is a unidirectionally tiltable variable displacement pump; a rod side selector valve that communicates a delivery port of the second hydraulic pump and the rod chamber with each other or interrupts the communication; an operation lever configured to instruct an action of the hydraulic cylinder; a cap pressure sensor that detects a pressure of the cap chamber; a rod pressure sensor that detects a pressure of the rod chamber; and a controller configured to control the first hydraulic pump, the second hydraulic pump, and the rod side selector valve based on inputs from the operation lever, the cap pressure sensor, and the rod pressure sensor; wherein the controller is configured to, in a case where a contraction action of the hydraulic cylinder is instructed through the operation lever, close the rod side selector valve to disable a rod assist action for supplying hydraulic fluid from the second hydraulic pump to the rod chamber when a differential pressure obtained by subtracting the pressure of the cap chamber from the pressure of the rod chamber is equal to or lower than a first threshold value set to a level equal to or higher than a selection setting pressure of the flushing valve, and open the rod side selector valve to enable the rod assist action when the differential pressure is higher than the first threshold value.