PURPOSE: To gradually increase the number of twists from a base end to an tip, by stripping off the sheath at a terminal part of wire material and setting this terminal part between a pair of rollers and drawing out this terminal part from between rollers when the terminal part is twisted by rollers.

CONSTITUTION: A terminal part 10 of a wire material is inserted between a pair of rollers 29 being located oppositely to each other and a core wire 11 of the wire material is held between rollers by bringing such rollers closer to each other and the rollers 29 are rotated in a same direction by a variable speed motor 30. In the next place, the rotating speed of rollers 29 is increased by gradually increasing the rotating speed of a variable speed motor 30 and the location of rollers 29 is moved to an axial direction of the wire material. With the aid of such serial operations, the core wire 11 at the terminal part 10 is twisted as being gradually increased in twist from the base end to the tip. Such alternate operation that the wire material be moved instead of moving a pair of rollers 29 is also applicable.