A seat belt retractor having a function of taking up a seat belt using a motor. A spool is fitted in a base frame such that a torsion bar functions as a shaft of the spool. Two gear-clutch casings are disposed on the left and right sides of the spool via a pretensioner and a locking mechanism, respectively, wherein two motors are disposed to the gear-clutch casings, respectively. The two motors achieve respective functions which are different from each other and rotate at respective constant speeds.