A passenger-protecting system includes a pressure sensor for detecting a pressure in an inner space of a door and a side airbag installed in the door. The pressure in the inner space increases when a side collision occurs. When the pressure in the inner space of the door exceeds a predetermined threshold level, the side airbag is inflated to protect a passenger from a side collision. There are certain situations where the operation of the side airbag is not necessary even if the pressure in the inner space of the door exceeds the threshold level. Such situations include: when the door is abruptly closed and a front airbag is inflated to protect a passenger from a frontal collision. According to the present invention, unnecessary operations of the side airbag are effectively avoided.