The method to obtain the maximum tilt and its azimuth for a single sheet of rock on which a single route of levelling survey is established may be applied to a levelling net of closed circuits. Of course, the azimuthal directions of combined line between 2 bench marks for a closed circuit cover far more directions than those for a single route. Accordingly, considerably reliable values of the maximum tilt and its azimuth can be obtained. In connection with the project of earthquake prediction, intervals of repeated levelling surveys tend to be shortened, and quantities of crustal movements are occasionally small. In such cases, reliable contour lines of movements can not be drawn, but secular change of the maximum tilt and its azimuth can be used as the monitor of the movement of the earth's crust.