PURPOSE: To reduce the stretch qualities of a box-shaped section forward and backward by half as compared with a conventional L-type frame by compactly storing drive members and a driving force generating device in the box-shaped section nearly uniformly stretched forward and backward from the lower end of a sheath-shaped section in a reverse T-type frame.

CONSTITUTION: The support member 2 of a buckle 1 connected with a seat belt is guided by a frame 2 movably from the loose position of the seat belt to the stretch direction. Drive members 11, 12 connected to the support member 2 via a cord 10 and a driving force generating device 13 applying a driving force to the drive members 11, 12 to pull the cord 10 in emergency are provided on the frame 3. The frame of this seat belt stretching device is constituted of a sheath-shaped section 3a opened at the upper end and guiding the support member 2 and a box-shaped section 3b stretched forward and backward from the lower end of the sheath-shaped section 3a into a reverse T-shape. The drive members 11, 12 and the driving force generating device 13 are arranged on the box-shaped section 3b so as to cross the axial line of the sheath-shaped section 3a.