Abstract: A wearing condition of a seat belt assembly is detected while considering a difference in a preferred mode of the seat belt assembly between the presence and the absence of a personnel who is seated upon a seat. In the prior art arrangement, in which a mechanical coupling of a seat belt assembly is considered alone, an alarm may be given off in the absence of the coupling. This may result in giving off an alarm for a vacant seat on which no personnel is seated. Such inconvenience is completely eliminated in accordance with the invention in which a decision differs between the presence or absence of a seating. The relationship between a webbing and a personnel sitting on the seat is analyzed, whereby an anomaly in the wearing condition such as sitting over the webbing after coupling the seat belt assembly can be detected.