PURPOSE: To measure the distance of an article to be measured with high accuracy even when circumferential temp. changes, by calculating average sonic velocity between the article to be measured and a calibration plate on the basis of the measured value of the temp. sensor between both of them and correcting a distance measuring value.

CONSTITUTION: Ultrasonic pulses are emitted to an article 3 to be measured and a calibration plate 4 from ultrasonic transmitter-receivers 1, 5 and reflected from both of them to be inputted to receiving circuits 8, 11. In signal processing circuits 9, 12, the time intervals of a transmitting pulse and a receiving pulse are respectively converted to the distance between the transmitter 1 and the article 3 to be measured and the distance between the transmitter-receiver 5 and the calibration plate 4 to be sent to CPU17. The measured values of temp. sensors 13, 14 are converted to digital values by an A/D converter 16 to be sent to CPU17. The average sonic velocity between the article 3 to be measured and the calibration plate 4 is calculated on the basis of the temp. distribution between both of them. The distance (d) between the transmitter- receiver 1 and the article 3 to be measured can be calculated from said sonic velocity in CPU17 with good accuracy.