In a method of producing seamless steel tube from billet having a square or rectangular cross-section by the step of piercing a workpiece and rolling steps following the piercing step, each of such steps being conducted with a plug held by a mandrel toward the center of cross section of the workpiece while the workpiece is rolled by a pair of rolls with axes arranged horizontally. The piercing step includes guiding the workpiece by a plurality of pairs of guide rolls provided along the rolling line ahead of the rolling rolls as the billet is forced therethrough to maintain the billet in such a position that the hollow shell will not have any nonuniformity of wall thickness and piercing workpiece with a plug as the workpiece is pushed and inserted between the rolling rolls under the forced guidance by a pushing force working in the axial direction, and as the plug progresses into the center of cross section of the workpiece, so as to pierce the workpiece, thereby producing seamless steel tube having very little nonuniformity of the wall thickness.