PURPOSE: To accurately measure by forming a second moving element having a distance sensor as a parentage type, partly duplicating a range capable of measuring, correcting obtained measured distance by a least squares method, and obtaining an entire displacement of a surface to be measured.

CONSTITUTION: A first moving element 5 is horizontally movably guided via a pair of slide members 4 engaged with a first guide rail 3. A second moving element 7 is engaged with a second guide rail 6 and provided to be horizontally movable. A distance sensor 8 is mounted at the element 7 to measure a distance to a surface to bg measured. A first moving unit 11 moves the element 5 provided at a guide trestle 2 side, and a second moving unit is provided at the element 5 side to move the element 7. A calculation controller inputs a measurement signal from the sensor 8 to obtain a measured distance, and overlaps part of the surface to be measured by the sensor 8 at the time of moving the element 7, and so corrects the the measured distances of the measured overlapped parts as to superpose approximate curves by a least squares method.