PURPOSE: To obtain a round steel bar having a cross section excellent in out of roundness by forming a material by arranging two pairs of grooved rolls close to each other and the roll axis of respective pairs perpendicularly to each other behind the final finishing rolls of hot rolling, in manufacturing a round steel bar by hot rolling.

CONSTITUTION: In hot rolling a round steel bar, respective pairs of grooved rolls 5,5' and grooved rolls 6, 6' are held by a roll holder 14 placed behind finishing rolls 1, 1' through a nozzle guide 4 so that the roll axis 8, 8' and 9, 9' of respective rolls 5, 5' and 6, 6' are at right angles to each other. In this case, when the central angle of curvature of the rolls 5, 5' near the rolls 1, 1' is put as θ1, and that of the next rolls 6, 6' is put as θ2, the grooves are formed so that they satisfy θ1=40W140°, θ2>180°-θ1. By using these pairs of grooved rolls in forming a round bar, the cross section of an obtained round bar is excellent in the out of roundness.