PURPOSE: To keep off any backlash over a long period of time by constituting each gear tooth with a tooth part and a shaft part being engaged in a state of sliding contact with teeth in the specified range, and having this shaft part pressed-in a hole part via a bearing.

CONSTITUTION: Each gear tooth member 2 consists of a tooth part 21 and a shaft part 22 being engaged in a state of sliding contact with a tooth 11 in the specified range. This part 22 is pressed-in a hole part 31 via a bearing 32. In an interval between this bearing 32 and the shaft part 22, there is provided with an axial traveling allowance in a closely fitted state. A compression spring 34 is interposed between a bottom part of the hole part 31 and a tip of the shaft part 22. In a state that the gear tooth member 2 and a worm 1 are engaged with each other, the most advanced position of the gear tooth member 2 is set up so as to have the member 2 slightly thrust in the axial direction. Thus, any backlash is preventable for a long period of time.