A spindle (2) of a seat belt retractor is rotationally driven when required by a motor (5) to the webbing take-up direction side through a power transmission mechanism section (6). Further, the spindle (2) is always connected to a take-up spring (7) and is always subjected to rotational power acting to the webbing take-up side. The structure enables a user wearing a seat belt to be restrained even in failure, realizing a safer and more reliable restraining device.