PURPOSE: To enlarge a detection range while performing the detection of a distance with high accuracy, by providing a sonic wave generator and a calibrating receiver spaced apart from said sonic wave generator over a constant distance to compare the sonic wave transmitting time between primary and secondary machines with that to the calibrating receiver.

CONSTITUTION: The sonic wave transmitting time from the sonic wave generator 11 and the calibrating receiver 12 of a primary machine 1 is compared with that to the sonic wave receiver 19 of a secondary machine 2 to enable the measurement of the opposed distance between the primary machine 1 and the secondary machine 2 under an arbitrary atmospheric condition without receiving influence of a temp. or air density. In the next time, when the sonic wave generator 11 is exactly opposed to the sonic wave receiver 19, sensitivity becomes highest. Hereupon, both turntables 4, 14 are rotated and a rotary angle highest in sensitivity is read by a potentiometer 18 and the distance detection range of the secondary machine 2 can be enlarged by a sonic wave receiving signal at the time of this rotary angle.