To simply predict and determine whether there is such abnormality that coating substance, including base rock or sediment coating an undersea tunnel, into which ground water containing sea water leaks, is eroding.

Measurement points A to J of the concentration of sodium ions as sea water deriving component are provided in the undersea tunnel. When some of the measurement data at each measurement point A to J are in disagreement for the data obtained by averaging the measurement data at each point in the past and if the disagreement is at every measurement point, it is determined as being normal, taking into consideration the factor of the environmental change of the region concerned as a while. If some of the measurement points are in disagreement, it is predicted and determined that abnormality is occurring at the measurement points of the disagreement.