A seat belt retractor comprising: a spool on which a seat belt is taken up; a motor that generates a driving torque for rotating the spool; and a power transmission mechanism that transmits the driving torque of the motor to the spool via a planetary gear mechanism, wherein the planetary gear mechanism includes a sun gear to which the driving torque of the motor is transmitted, a rotatable internal gear member including inner teeth formed along an inner circumferential surface and ratchet teeth formed along an outer circumferential surface, a predetermined number of planet gears each meshed with both of the sun gear and the inner teeth of the internal gear member, and a carrier that rotatably supports the predetermined number of planet gears and transmits the driving torque of the motor to the spool, characterized in that the seat belt retractor further comprises: the power transmission mechanism includes a clutch pawl set to move between a non-operating position where the clutch pawl is disengaged from the ratchet teeth of the internal gear member thereby allowing the internal gear member to rotate thus disconnecting a power transmission route and an operating position where the clutch pawl is engaged with one of the ratchet teeth of the internal gear member thereby inhibiting the internal gear member from rotating thus connecting the power transmission route, the internal gear member is formed of a noise suppressing material that suppresses noise generated when the planetary gear mechanism is operating, and the seat belt retractor further includes a deformation restricting portion that restricts deformation of the internal gear member 24), the deformation restricting portion is provided on the carrier, the carrier includes an annular portion, and the deformation restricting portion is formed in an annular shape along the annular portion of the carrier so as to restrict deformation of the internal gear member when the internal gear member about to be deformed makes contact with the deformation restricting portion. a spool on which a seat belt is taken up; a motor that generates a driving torque for rotating the spool; and a power transmission mechanism that transmits the driving torque of the motor to the spool via a planetary gear mechanism, wherein the planetary gear mechanism includes a sun gear to which the driving torque of the motor is transmitted, a rotatable internal gear member including inner teeth formed along an inner circumferential surface and ratchet teeth formed along an outer circumferential surface, a predetermined number of planet gears each meshed with both of the sun gear and the inner teeth of the internal gear member, and a carrier that rotatably supports the predetermined number of planet gears and transmits the driving torque of the motor to the spool, characterized in that the seat belt retractor further comprises: the power transmission mechanism includes a clutch pawl set to move between a non-operating position where the clutch pawl is disengaged from the ratchet teeth of the internal gear member thereby allowing the internal gear member to rotate thus disconnecting a power transmission route and an operating position where the clutch pawl is engaged with one of the ratchet teeth of the internal gear member thereby inhibiting the internal gear member from rotating thus connecting the power transmission route, the internal gear member is formed of a noise suppressing material that suppresses noise generated when the planetary gear mechanism is operating, and the seat belt retractor further includes a deformation restricting portion that restricts deformation of the internal gear member 24), the deformation restricting portion is provided on the carrier, the carrier includes an annular portion, and the deformation restricting portion is formed in an annular shape along the annular portion of the carrier so as to restrict deformation of the internal gear member when the internal gear member about to be deformed makes contact with the deformation restricting portion. the power transmission mechanism includes a clutch pawl set to move between a non-operating position where the clutch pawl is disengaged from the ratchet teeth of the internal gear member thereby allowing the internal gear member to rotate thus disconnecting a power transmission route and an operating position where the clutch pawl is engaged with one of the ratchet teeth of the internal gear member thereby inhibiting the internal gear member from rotating thus connecting the power transmission route, the internal gear member is formed of a noise suppressing material that suppresses noise generated when the planetary gear mechanism is operating, and the seat belt retractor further includes a deformation restricting portion that restricts deformation of the internal gear member 24), the deformation restricting portion is provided on the carrier, the carrier includes an annular portion, and the deformation restricting portion is formed in an annular shape along the annular portion of the carrier so as to restrict deformation of the internal gear member when the internal gear member about to be deformed makes contact with the deformation restricting portion.