To detect the change in ground directly by laying a composite multicore cable, wherein a plurality of single-core wires having the different elongation rates respectively are combined so as to cross a monitoring surface, and laying the wire simply.

A composite multicore cable (CN cable), wherein a plurality of single-core wires (S wires) having the different elongation rates respectively are combined, is laid so as to cross a monitoring surface. This CN cable, which is laid so as to cross the estimated dangerous parts, is fixed at two or more parts. A minute current is carried by each laid S wire, and an electric closed circuit is formed. When the distance of the interval between the fixing parts of such CN cable is expanded by any cause, each S wire is cut in the sequence of the smaller elongation rates with the cutting-time difference being generated. A detecting part, which is connected to each S wire, detects the stopping of carrying current and indicates the specified signal assigned to the cut S wire to the outside. Thus, the motion of landslide can be monitored even by a person without expertise.