A worm gear mechanism includes a worm, a torque-transmitting worm wheel meshing with the worm, and an auxiliary worm wheel disposed concentrically with the torque-transmitting worm wheel. The auxiliary worm wheel has a pitch circle diameter set to be greater than a pitch circle diameter of the torque-transmitting worm wheel and hence meshes with the worm in a meshing phase different from that of the torque-transmitting worm wheel. A screw thread provided on the worm actually meshes with teeth of the torque-transmitting worm wheel along a length defined as an actual meshing length. The screw thread has an effective length reduced to be approximately equal to the actual meshing length.