A backlash adjustment mechanism includes a first cylindrical gear fixed in an axial position, a second cylindrical gear moveable along the axis and meshed with the first cylindrical gear, and an elastic member biasing the second cylindrical gear towards the first cylindrical gear. The addendum modification coefficients in the first and second cylindrical gears are linearly changed along their axes, respectively, an end of the first cylindrical gear with higher addendum modification coefficient meshes with an end of the second cylindrical gear with correspondingly lower addendum modification coefficient, and an end of the first cylindrical gear with lower addendum modification coefficient meshes with an end of the second cylindrical gear with correspondingly higher addendum modification coefficient.