Satellite sensors (16, 18) that output level signals corresponding to an impact on a vehicle (10) are provided right and left portions of a forward portion of the vehicle (10), separately from a floor sensor (14). If a map (Lo1, Lo2, Lo3, Hi) selected as a threshold changing pattern from an impact (deceleration) detected based on the output signal of one of the satellite sensors (16, 18) and a map (Lo1, Lo2, Lo3, Hi) selected as a threshold changing pattern from an impact (deceleration) detected based on the output signal of the other satellite sensor are different from each other, the map of a smaller threshold is selected from the maps based on the output signals of the two satellite sensors (16, 18), and is set as a threshold changing pattern for determining whether to activate an airbag apparatus (30).