PURPOSE: To generate an alarm in the proper distance between cars to the following car, by arithmetically comparing the signal of a distance detecting means to a rear obstacle with the signal of a car speed detecting means through an arithmetic means and operating an alarm device through a driver.

CONSTITUTION: Firstly a car speed detecting means 2 operates such that a signal of an engine speed detecting unit is corrected in a change lever position and converted into a car speed then input to an arithmetic means 3. While an ultrasonic distance detecting means 1 detects a distance to a rear obtacle, that is, the distance between cars and inputs the distance to the means 3. Here the both signals are arithmetically compared to generate an output to an alarm device 5 through a driver 4. Accordingly, the device 5 successively turns on alarms 51, 52, 53 when the distance between cars becomes, for instance, 10m or less, 8m or less, 6m or less at 100Km/H car speed, while successively turns on the alarms 51W53 when the distance becomes 6m or less, 4m or less, 2m or less at 500Km/H car speed. In this way, an accident of collision from behind can be effectively prevented.