PURPOSE: To detect an object precisely by dividing the detection range of an ultrasonic sensor into a near distance range and a remote range, fractionalizing the displacement range of the sensor for remote detection, and heightening the reception gain and transmission frequency.

CONSTITUTION: A microcomputer 30 divides the detection range of an ultrasonic sensor 11 into a near and a remote ranges using the 1/2 value of the maximum range as a standard and judges the measuring range at the time. To sense a remote obstacle as far as 1/2 of the maximum measurable distance or farther, the rotation angle of the sensor 11 is fractionized. Moreover, to sense an object at a far distance, the transmission frequency of the sensor 11 is increased by a prescribed extend and the reception gain is also increased by the prescribed extent to heighten the gain of the echo signal to be received. Furthermore, to set the reception time of the echo signal to be longer, the transmission time of the ultrasonic waves in one cycle of transmission and reception is made short and the reception time is made long. That is, the duty ratio D at the time of transmission and reception of the one cycle of the transmission of the reception is decreased by decrease γ and thus the obstacle at a remote position can be more precisely detected.