PURPOSE: To make positioning speedy and easy by rotating a flat surface on one side with regard to an azimuth to minimize an output signal from corresponding first and second level detecting means.

CONSTITUTION: Two pairs of sensors S and S' are calibrated by putting them on the same display (the surface 13 of a fixing base 12), etc., so that corresponding inclinometers I1/I2/I1/I2' are positioned to be mutually in a collinear relation and an attitude difference between the corresponding inclinometers is adjusted. Next, S' on one side of the two pairs of sensors is removed from the surface 13 of the fixing base 12 to be placed on the surface 15 of a fixing base 14 to position the corresponding interferometers of two pairs of sensors S and S' in a collinear relation. Finally the two fixing bases 12 and 14 are tilted by the same quantity and a fixing base on an adjusted side is fixed with regard to an azimuth to make a differential signal zero to make the slave fixing base 12 coincide with the master fixing base 14 concerning a rolling, a pitching and the azimuth.