PURPOSE: To prevent flawing on the inside and outside surfaces of a seamless steel pipe in a mandrel mill for producing said pipe by regulating the bulging width of a pipe material in the stage of drawing in an adequate range.

CONSTITUTION: A hollow blank material 14 inserted therein with a mandrel bar 16 is successively rolled down with caliber rolls 17 in each stand of a mandrel mill 15. The rolling reduction and revolving speed of each roll 17 are set respectively by a screw 31 and a main motor 32, and are made adjustable by controllers 33, 34. A sensor 35 for bulging width is provided on the outlet side of the prescribed, for example, 3rd stand and the detection signal thereof is analyzed with a measurement device 36 for the bulging width, by which the bulging width BX of the actual pipe material is measured and is transmitted to a main arithmetic device 37. The adequate bulging width BO on the outlet side of the 3rd stand is transmitted as the set value of a setter 38 to said device and the adjusted rolling reductions and adjusted roll rotating speeds in the 1st W the final stands necessary for making the deviation between the value BX and BO zero are stored therein. The adjusted rolling reduction and revolving speed of each stand that make the deviation, if any, zero are then transmitted to the controllers 33, 34, whereby the control is accomplished.