PURPOSE: To make it possible to form uniform grooves on the external surface of a soft tube by providing a guide for determining a position for the soft tube.

CONSTITUTION: A guide 6 is concentrically installed upright in a hole la where a soft tube 10 of a tool body 1 is introduced, using a screw. A cutter is forced down resisting a spring 4 to lift up the blade of the cutter 2, then the inner diameter of the soft tube 10 is introduced into a guide 6 and the cutter 2 is allowed to become engaged with the external periphery of the soft tube 10 by the force of the spring 4. The soft tube 10 and the grooving tool are rotated in a backward direction until the cutter 2 comes in contact with a stopper 5 to form a groove. During groove formation, the determined positions of the grooving tool and the soft tube 10 become stable, then the depth of the formed groove becomes fixed and the thickness variations become almost entirely eliminated.