A cooling system that cools hot rolled long steel bar, provided with a plurality of chambers that are arranged along the longitudinal direction of the rolled steel bar. Each of the plurality of chambers is provided with a blow outlet that, facing from the chamber to the rolled steel bar, blows out compressed air for cooling that is introduced to the chamber from a gas inlet that is connected to the chamber; a nozzle plate having a plurality of nozzle holes that is provided at this blow outlet so as to face the rolled steel bar; a cooling water supply nozzle that supplies cooling water into the chamber; and a rectifying plate that is provided between the gas inlet and the cooling water supply nozzle, and that prevents the compressed gas for cooling that is introduced from the gas inlet from directly striking the nozzle plate. The cooling system of the present invention sprays a cooling medium that is produced by mixing the cooling water that is supplied from the cooling water supply nozzle and the compressed gas for cooling that is introduced from the gas inlet and rectified by the rectifying plate toward the rolled steel bar through the nozzle holes of the nozzle plate, and performs uniform cooling of the surfaces of the rolled steel bar.