To detect any abnormality in an OF cable line, based on the result of a gas-in-oil analysis with accuracy by calculating the ratio of the concentrations of a plurality of specific gases in the insulating oil of the OF cable to obtain measured graphic patterns, and comparing the patterns against a basic graphic pattern.

Gases in the insulating oil of an OF cable are analyzed to determine the concentrations of hydrogen, acetylene, ethylene, methane, carbon dioxide, and carbon monoxide. Four types of gas concentration ratio are selected, and these concentration ratio are plotted on a radar chart, using rectangular coordinates comprising x- and y-axes to determine the correlation between the gas concentration ratios and abnormal phenomena. Meanwhile, based on the ratios of gas concentrations in various abnormal phenomena, the types of abnormal phenomena are correlated to the gas concentration ratios and stored together with a basic graphic pattern in advance. Measured graphic patterns are checked compared and compared with against the basic graphic pattern for detecting the occurrence of any abnormality. Thus, abnormality in the OF cable is detected exactly.