An activation control apparatus of an air bag system of a motor vehicle includes a first sensor disposed at a predetermined position within a vehicle body, for generating a signal indicative of an impact applied to the vehicle, and at least one second sensor disposed frontwardly of the position of the first sensor, for generating a signal indicative of an impact applied to the vehicle. The control apparatus is operable to activate an air bag device when a parameter determined based on the output signal of the first sensor exceeds a predetermined threshold pattern that is determined based on the output signal of the second sensor. When the predetermined threshold pattern is changed from a reference pattern to a desired threshold pattern that provides lower threshold values, the pattern is changed step by step at predetermined time intervals, without skipping an intermediate pattern or patterns between the reference pattern and the desired pattern. |