PURPOSE: To engage a passive belt with an appropriate part of a human body for safety, by automatically shifting the position of a hook for engaging the passive belt, depending on the size of the human body.

CONSTITUTION: When a seat 4 is displaced back or forth, the position of the seat 4 is detected by a sensor 5. An internal motor is driven depending on the detected position to move a hook 3 back or forth to change the gradient angle of the passive belt 1. According to this constitution, the position of the passive belt 1 can be adjusted depending on the size of a human body to carry out a comfortable and safe drive.