To detect a collision efficiently and rapidly.

A front piece 36B of a hinge 36 is fixed to an upper end rear part 28A of a hinge retainer 28 fixed to a longitudinal wall part 12A of a center pillar, with the longitudinal wall part 12A of the center pillar held in between, and a sensor 14 is fixed to a lower end front part 28B of the hinge retainer 28. A rib 50 and cutouts 52, 54 are formed at the nearly center part in the longitudinal direction of the hinge retainer 28. When rotational force A inward in a cabin acts upon the upper end rear part 28A of the hinge retainer 28, the upper end rear part 28A of the hinge retainer 28 easily bends inward in the cabin with the rib 50 and the cutouts as the starting points, and acceleration G generated to the sensor 14 is prevented from being substantially reduced by the rotational force A.