Seat belt retractor

To provide a large reduction ratio to obtain torque enough for winding a seat belt without increasing the size. A retractor 1 comprises a spool 4 for winding a seat belt, a motor 6 for generating driving torque to rotate the spool 4, and a power transmission gear mechanism 8 for transmitting the driving torque of the motor 6 to the spool 4. The power transmission gear mechanism 8 is provided with a hypocycloid gear mechanism for reducing the rotational speed of the motor 6 while transmitting the rotation of the motor 6 to the spool 4 (Fig 1).