To limit the maximum speed of a vehicle without changing an existing engine control unit.

A car speed limit device 4 is attached externally to the existing engine control unit 3, and when the car speed reaches a preset. maximum speed, an acceleration signal is processed so that the car speed does not exceed the preset maximum speed, and the processed signal is given to the existing control unit 3 as an acceleration signal. The engine is, therefore, controlled by eh control unit 3 based on the processed signal so that the car speed does not exceed the preset maximum speed. The car speed can thus be limited without changing the existing control unit 3.