An obstacle detecting means for detecting data related to the position of an obstacle existing on the side of a vehicle or in front thereof, a computer for computing the detected data, and an alarming means are mounted in the vehicle. The computer includes: a function of setting an alarming area having a certain distance from the side part of the vehicle and a forwardly extended line of the side part, in an outward direction lateral to the side part; a function of tracking the obstacle based on the obstacle data; a function of calculating a traveling locus of the obstacle relative to the vehicle and predicting the traveling locus thereafter; a function of determining whether the traveling locus enters into the alarming area; and a function of generating an alarm to the occupants by the alarming means when the traveling locus is determined to enter into the alarming area.