PURPOSE: To enable the simple use of a substantial inertial sensor on a colliding section of a vehicle by making a deceleration sensor movable to a mounting bracket by the collision of the body structure.

CONSTITUTION: A cylindrical inertial sensor 10 is tightly pressed in a complementary bracket 12 in such a relational state as to slide relatively to the bracket 12. This sensor 10 is protruded forwards from the bracket 12 so as to encounter the crush of a body structure, i.e., the collision before the bracket 12. The bracket 12 is fixed on the panel 14 of a vehicle (not shown in Fig.) with screws 16. A pigtail 18 is provided with an electric connector 20, extended through an opening 22 formed on the body panel 14 and connected to a regular air-bag expansion device (not shown in Fig.).