In order that a worm 1 is rotated, a clearance (a region constituting a clearance portion between bearings and a worm gear) is formed (FIGS. 4(1) to (3)) in an upper surface of a base plate 3. A mold 12 for forming a lower half region of the worm therein is formed (FIG. 4(8)). A material 5 for the worm is deposited (FIG. 4(9)) to a height equal to that of a lower half region plus that of an upper half of the worm. An upper half of the worm is formed (FIGS. 4(10) to (12). Finally, the material 13 in the clearance between the base plate 3 and gear 1 is removed (FIG. 4(13)).