To provide the sensor device, which controls the developing speed, degree and the like of an air bag so as to avoid the risks by the action of the air bag on occupants, especially small persons, by detecting the difference in body weights.

A layer 14 made of compressible material is arranged between two conductive layers 10 and 18. In one embodiment, the device is constituted so that the electric capacitance between the conductive layers 10 and 18 is measured and the fact that a body is mounted on the sensor. In the other embodiment, a penetrating opening is formed between one conductive layer and the other conductive layer in the compressible layer 14. The mutual contact is obtained by the elastic deformation of the layer formed on conductive fabric through the opening. The change in resistance value is detected by the formation of this short-circuit circuit, and the characteristics of the body are detected.