A load sensor detects a load of an occupant on a seat of a vehicle and provides a load signal. A determination section determines a physical size of the occupant by classifying the load signal into a subject class among a plurality of classes ordered from small to large based on threshold loads and performs an inter-class transition of the subject class from a transition source class into a transition destination class based on change of the received load signal. When (i) both the transition source class and the transition destination class belong to an airbag-inflating class that enables inflating of an airbag, and, simultaneously, (ii) the transition destination class is smaller than the transition source class and immediately adjacent to the transition source class, the determination section makes the determined class remain in the transition source class.