When a small-size dummy has collided against a vehicle and once the head of the small-size dummy hits the deployed external airbag at a pre-determined velocity, the inner pressure of the deployed external airbag is set to a characteristic such that the inner pressure takes a predetermined maximum level value capable of not only preventing the dummy from bouncing off the airbag but also preventing the head of the dummy from hitting the bottom of the airbag. When a medium-size dummy has collided against the vehicle, the inner pressure of the deployed external airbag is set to a characteristic such that the inner pressure decreases away from the maximum level value at a predetermined decrease rate to prevent the medium-size dummy from bouncing off the airbag.