PURPOSE: To shorten the line length of a steel tube producing device by arranging a three roll forcibly drive system stand in series in a tube shape finishing device to finish-shaping it into a required diameter.

CONSTITUTION: A strip 1 on a pass line is fed from a coil guide 48 to a forming roll stand 30, it is shaped into a tube shape like during passing though plural forming roll stands 30, and reaches to a seam welding device 2. The electro- resistance-welded tube, whose unneccessarily build up welded part is cut with a bead cutter 31, is taken out from a pull-out stand 32 to a cooling device L. It is cold in the cooling device L, and reaches to a finishing device A. One driving motor 51 drives plural stands (for example two stands) of three roll forcibly drive system stand or four roll forcibly drive system stand. Therefore, the total length of device can be made shorter than the length of device that 6 to 8 stands of two roll stands are arranged in series.