PURPOSE: To form a groove high in dimensional accuracy by assuming a position facing a cutter on the external peripheral surface of the tube body based on a detected position by a position detecting means arranged on both sides of the cutter in the peripheral direction of the tube body and adjusting the position of the cutter to a prescribed groove depth.

CONSTITUTION: The tube body is rotated round the axis to perform the edge preparation by the cutter to its external surface. Concerning the position to perform the edge preparation on the external peripheral surface 1 of the tube body, position detecting means 5a, 5b are arranged at the positions on both sides of the cutter 3 in the peripheral direction of the spiral steel tube 1 to detect the height and the position on both sides of the cutter 3 in the peripheral direction of the spiral steel tube 1. Though a sectionalal shape of the external peripheral surface 1 of the tube body is trained in some degree, no large error is made by exerting the direct influence on detection of a working position. The position of the cutter is adjusted so that a prescribed groove depth is obtained to the position of a part facing an assumed cutter. In this way, a groove high in dimensional accuracy can be formed.