To allow the section shape of a seamless steel tube to be completely round and to straighten the bend thereof by using a rotary sizer whose roll shape is deformed uneven as the rotary sizer in a process for improving the roundness.

The roll shape of a line contact type is uneven. A seamless tube 2 is subjected to the bending in the longitudinal direction between the line contact type rolls 6, and a bending surface is rotated according to the feed to straighten the bend. The roll shape is set so that the roll intervals of a passage, where the seamless steel tube 2 is brought into contact with the line contact type roll 6 and passed are equal to each other, and the roundness is improved thereby. The plane of the line contact type roll 6 has a curvature, preferably of 1-5°, the angle of inclination of the axis of each line contact type roll 6 is set at 10-20°, and the line contact type roll is inclined in the direction opposite to the advancing direction of the seamless steel tube 2.