A seat 1 has a seat cushion 2 and a seat back 3 and is mounted on a vehicle such as a car. A reclining mechanism 4 capable of adjusting the tilt angle of the seat back 3 is provided between the seat cushion 2 and the seat back 3. The reclining mechanism 4 has a seat driving device 21 provided with an electric motor 22 and a speed reduction mechanism 23. The speed reduction mechanism 23 has a worm shaft 45 connected to a motor shaft 34 of the motor 22 and a worm wheel 47 meshed with a worm 46 of the worm shaft 45. The worm 46 is a multi-thread worm having two or more threads.