PURPOSE: To improve safety of a automobile in travelling by mounting a detecting member moved by inertia forward and backward in response to change in speed of the automobile in a housing on the car body to detect and indicate the change in speed for transmitting quickly the speed change to the other vehicles.

CONSTITUTION: Two rollers 1, 2 are arranged movably and rotatably in two guide grooves 3, 4 arranged parallel to each other. Frame members 5, 6 constituting the guide grooves 3, 4 are respectively provided on a base plate 7 fixed to a car body. Also, above the rollers 1, 2 are respectively provided indicatting plates 8, 9 pivotably connected through hinges 11, 12 to an erect support plate 10 provided at the frond end of the base plate 7. Further, the roller and the indicating plate 8 are interconnected through a connecting rod 13 provided on the upper end with a projection 15 and on the lower end with a shaft 14. On the other hand, in a center hole 2' in the roller 2 is fitted a U-shaped shaft 17 so that the indicating plate 9 can be erected by moving a projection 17' along with the rolling of the roll 2 to collide with a operating rod 18.