A triggering mechanism for a side airbag has a deformation sensor for determining a rate of deformation. The deformation sensor is made up of a total of two pressure-sensitive sensor elements, which are attached on a firm carrier plate parallel to the outer skin inside the vehicle. Spacer elements support the carrier plate against a structural element running parallel to the outer skin. The pressure-sensitive surface of a first sensor element in this case faces the outer skin and the pressure-sensitive surface of a second sensor element faces the structural element. The spacer elements are designed, for example, using predetermined buckling points, clip elements, etc., in such a manner that they withstand a pressure on the support plate only up to a threshold and, if the threshold is exceeded, yield abruptly. This is so that, in the event of a deformation-induced force effect, the sensor elements respond one after the other. An evaluation circuit checks the time interval between the two sensor signals and, below a critical time, sends a triggering signal to the side airbag.