PURPOSE: To prevent rear-end collision by calculating the inter-car safety distance from the distance from the own car to a car or any other object ahead or behind and the own car running speed, and controlling related working devices according to the result from calculation.

CONSTITUTION: This device includes an ahead car distance and a behind car distance measuring instruments 11, 12 and a car speed measuring instrument 2, and the sensing signals from them are inputted into a microcomputer 3. A safety distance which requires a deceleration or brake application is calculated by the safety distance calculating mode set 30 in the microcomputer 3, and also the safety distance for behind the car is calculated. The specified calculations are conducted by the brake lamp control mode set 31, deceleration/ braking alarm indication mode set 33, cruising device control mode set 34, and start run alarm indication mode set 35. The operations of the working devices 4 such as a brake lamp actuator 41, deceleration requiring and brake alarm indicators 421 and 422, headlight 43, etc., are controlled in accordance with the outputs of the respective sets 30-35.