PURPOSE: To shorten the operating cycle of an ultrasonic sensor as well as to make the positional detection of an obstacle to be conducted accurately, by operating plural ultrasonic sensors simultaneously with every interval proportionate to a measurable distance of each ultrasonic sensor.

CONSTITUTION: In the case where this system is put into operation to detect an obstacle in the way of the left side of a large-sized vehicle 1, ultrasonic sensors 2 are set up at regular intervals on the left side of the large-sized vehicle 1, controlling each of them by a processing unit 3, and the detected obstacle is displayed by a display unit 4. The start of this sensor 2 is to cause these ultrasonic sensors 2 to operate at the same time, which are set up at the following intervals that an ultrasonic sensor 2B is beyond the range of the maximum detection distance of an ultrasonic sensor 2A, an ultrasonic sensor 2C beyond the range of the maximum detection distance of said sensor 2B and so forth. Likewise the operating interval of the sensors 2 to be simultaneously operated should be set so that the reflection wave receiving voltage satisfies the product of values derived from a function of the transmitting voltage, reflection coefficient and measuring distance, and a directional function of sensors 2.