PURPOSE: To suitably control the length of a steel tube through a sizing mill of the final stage of rolling by fixing stands on the final outlet sides of plural sizing mill stands to the maximum defined number of rotation and making the numbers of rotations of other stands variable.

CONSTITUTION: One of the sizing mill in the production process of a seamless steel tube or a stand on the outlet side or the inlet side constituting a stretch reducer is considered a fixed reference point to control the number of rotations. In the control of its length, when the set value of the number of rotations on the variable side obtained by a length control model exceeds the upper threshold of equipment or the threshold determined by the operating conditions, an excessed set number of rotations added to or subtracted from a set number of rotations on the fixed side to hold the inclination of a number of rotations at each stand from the inlet side to the outlet side.