A retractor for a seat belt is disclosed, including a frame, a spool that is mounted rotatably to the inside of the frame, a webbing that is wound on the spool to be pulled-in constantly to the inside of the frame and pulled-out freely from the inside of the frame to the outside thereof, a controller for recognizing vehicle information and controlling the pulling-in of the webbing, a motor for generating a driving force to rotate the spool in accordance with the control of the controller and a clutch for receiving the driving force from the motor and transmitting the driving force to the spool wherein the clutch includes; a driven gear that receives the driving force from a driving gear provided on a rotation shaft of the motor; a plate that is provided facing to one end of the driven gear and inter-connected with the driven gear; a braking disk that is provided to be engaged with the other face opposing one face of the plate, facing to one end of the driven gear, and is in friction with the plate to stay temporally the plate when the plate is inter-connected with the driven gear; and a locking pole that is mounted pivotably to the plate and pivoted by a rotation of the driven gear when the plate is stayed temporally by the braking disk to be meshed with a pinion gear provided on a rotation shaft of the spool thereby transmitting the driving force transmitted from the driven gear to the spool.