A flat spring of a vehicular seat is elastically supported by means of supporting springs that extend in accordance with the load of an occupant. The seat has displacement sensors, first-stage determination step, second-stage determination step, and third-stage determination step. The displacement sensors output electrical signals corresponding to the respective elongations of the springs. The first-stage determination step determines the occupant in the seat in accordance with voltage values delivered from the displacement sensors. The second-stage determination step determines whether or not the result of determination by the first-stage determination step is maintained for a given period of time. The third-stage determination step compares the result of determination by the second-stage determination step with the preceding result of determination and specifies the occupant in accordance with the result of the comparison.