To highly precisely detect a obstacle by preventing the oscillating waves of sensors from interfering each with other between vehicles performing follow-up traveling.

When plural vehicles 101, 102, and 103 which are provided with sensors (for example, ultrasonic sensors) 4 and 5 at the front part and rear part respectively are performing follow-up traveling in one row as shown by a figure (a), the backward and forward traveling directions of each vehicle 101, 102, and 103 are detected, and only the ultrasonic sensors 4 corresponding to the traveling directions are operated. Thus, ultrasonic waves oscillating from the rear ultrasonic sensors 5 and the front ultrasonic sensors 4 of the vehicles performing the follow-up traveling can be prevented from interfering each with other, and the erroneous detection of an obstacle can be prevented. When the ultrasonic sensor 4 of the following vehicle 102 breaks down as shown by a figure (b), the rear ultrasonic sensor 5 of the leading vehicle 101 is operated.