PURPOSE: To make it possible to always accurately measure a distance even if the level of a receiving signal is changed, by correcting a measured distance by detecting the receiving peak value of a reflected ultrasonic wave from an object.

CONSTITUTION: The peak value of the reflected ultrasonic wave from an object received by an ultrasonic receiver is detected by a peak value detecting means and the distance to the object measured by a distance measuring means is corrected on the basis of the correction value due to a measured value correcting means corresponding to the peak value. By this correction, for example, when a receiving level is lowered, it is prevented that the measured distance due to a delay time until the receiving level exceeds a threshold value becomes longer than an actual distance and, even if the level of a receiving signal is changed, distance measurement can be performed always accurately.