A motorized seat belt system includes: a spindle around which webbing is wound; an output gear connected to the spindle through a power transmission shaft; a torsion portion provided on the power transmission shaft or the output gear; a motor which transmits rotational power to the spindle; first and second reduction gears which are connected to an output shaft of the motor and transmit rotational power of the motor to the output gear; a first rotation amount detecting sensor that measures a rotation angle and a rotation direction of the power transmission shaft; a second rotation amount detecting sensor that measures a rotation angle and a rotation direction with respect to a rim portion of the first reduction gear; and a controller which controls output of the motor based on detection signals from the first rotation amount detecting sensor and the second rotation amount detecting sensor.