For the river structures, such as slice gates and slice pipes, constructed on the weak ground, structures often come out due to forced settlement of embankment, which results in the formation of cavity and loose ground. These deformations can cause disasters due to leakage by water channel and fracture of bank. It is significant to detect and grasp these situations in early stage. In this study, resistivity imaging was conducted around slice gate in several times, to estimate the loose region based on the difference in the obtained resistivity data. Resistivity was measured in two times with a certain interval using the same line. The loose region around the slice gate was estimated from the change of saturation condition and the saturation line by considering the difference. Even in the case when judgment was hard by a single measurement, it was possible to estimate the loose region based on the difference from multiple measurements. 3 refs., 6 figs.