PURPOSE: To properly attract driver's attention by putting an obstacle detecting means in operation when it is confirmed that a blinker is in operation and generating an alarm when an obstacle is detected slantingly behind a vehicle.

CONSTITUTION: When a decision means 9 decides the operation state of the blinker 8, an obstacle detecting means 6 which detects the obstacle positioned slantingly behind the vehicle during the operation of the blinder 8 is put in operation. Then when the blinker 8 operates at the time of a course change where this vehicle possibly contacts the obstacle such as a bicycle, the obstacle detecting means 6 detects the obstacle positioned at the blind part of the rear visual field and an alarm part 7 generates an alarm. Consequently, driver's attention is attracted to improve the safety of the vehicle.