PURPOSE: To prevent a seat belt from being abruptly locked upon pulling thereof by providing a means for detecting the wearing condition of the seat belt, and a means for negating a mechanism for sensing an acceleration upon pulling of a webbing when the seat belt is not yet worn.

CONSTITUTION: When a passenger is fastening a seat belt, a switch 32 is turned off since a tumbler 30a is not yet fitted in a buckle 31, and accordingly, a solenoid 34 is deenergized so that a lock plate 36 is pushed in by the pulling force of a spring 38 through the intermediary of an arm 39, overcoming a spring 37. Further, a projection 36a on the lock plate 36 enters into a gap other than a part occupied by a pin 4d on a latch frame 4 within a rectangular aperture 35a in a sensor ratchet 35 so that the sensor ratchet 35 is prevented from rotat ing relative to the latch frame 4. Accordingly, the lock plate 36 is not subjected to rotation inhibiting action by a sensor cover 11, thereby a webbing sensing mechanism in negated.