A worm 1 is composed of an annular tooth body 11 having a tooth 10 on the outer surface thereof, a core body 12 inserted inside the annular tooth body 11; and an elastic coupler 13, which can be transformed in the radial direction, for coupling the core body 12 with the annular tooth body 11. The worm 1 is engaged with the worm wheel with the elastic coupler 13 being transformed in the radial direction in such a manner that a negative amount of backlash is generated at the engagement portion. This can be also employed for the worm wheel. With a reduction gear combination having this structure, the backlash amount at the engagement portion can be adjusted to a proper amount without selecting elements such as the worm and the worm wheel to be assembled.