PURPOSE: To prevent people aboard from taking a whiplash injury, by letting a lock for a reclining back come off automatically and set an individual free simultaneously with a rear-end collision by a succeeding car, while making each seat so as to be rotated at a proper angle.

CONSTITUTION: When a car is struck from behind, a bumper 1 causes some deformation whereby a part 2 supporting this bumper 1 is thrust in, producing a positional variation to some extent. At this time, a central part of length in a rod 3 is made so as to be rotatable to the desired degree as a center shaft 4, while one end 5 of the rod is connected to a part supporting the bumper, and another side end 6 is connected to a wire, while the said wire is made to be connected to a lock pin of a seat's reclaining back, thus the lock pin is released.