PURPOSE: To improve reduction of area and to easily form a square pipe by driving all of double rolls forcedly.

CONSTITUTION: Four rolls 11, 12, 13, 14 are arranged by 90°. A resistance welded tube is supported and formed by them. Then, two input shafts 24, 25 are arranged in parallel. A pair of bevel gears 15, 16 are arranged on the shafts 24, 25. The bevel gears 15, 16 are engaged with the double roll bevel gears arranged in the directions of 45° to the input shafts 24, 25. In the same way, bevel gears 19, 22 are turned. In this way, all of four rolls 11, 12, 13, 14 are driven forcedly. Consequently, it is possible to increase the pressure given by the driven rolls 11, 12, 13, 14 to the resistance welded tube and the corner parts can be formed sharply in reducing of the square pipe.