A floor sensor (14) is disposed at a center portion of a vehicle body (10) and front sensors (16, 18) are disposed at front portions of the vehicle body (10). An airbag apparatus (30) always activates when a value determined by the relationship between a floor deceleration (GF) detected by the floor sensor (14) and a change in floor velocity (Vn) exceeds a High Map toward the high side of the floor velocity. The airbag apparatus (30) also activates on the condition that the value determined by the relationship between the front deceleration (Gs*) detected by the front sensors (16, 18) and the change in floor velocity (Vn) exceeds a Front Map toward the high side of the floor deceleration (GF) when the value determined by the relationship between the floor deceleration (GF) detected by the floor sensor (14) and the change in floor velocity (Vn) exceeds the Low Map but does not exceed the High Map toward the high side of the floor velocity.