PURPOSE: To improve productivity and quality with a cross helical rolling mill for producing a seamless steel pipe by measuring the eccentric motion of a blank pipe during rolling, detecting the rate and position of end bending and decreasing the rate of end bending of the final product.

CONSTITUTION: The 1st distance (l) up to the circumferential surface of a blank pipe 6 during rotation and the time TL until the amplitude of the measuring data of the 1st distance (l) attains a prescribed value or below are measured by using a laser rangefinder 8, etc. on the outlet side of a cross helical rolling mill. The rate M of end bending at the top end of the pipe 6 is calculated from such measuring data by using the equation I . The 3rd distance (l) from the position where the top end of the pipe 6 begins to bend up to the center of the pierced hole is calculated by using the equation II. The calculated value is fed back to the pipe 6 to be inserted next so that the trouble to be generated in the later stage is decreased.