Recently, some accidents which are caused by external corrosion of pipes often occur in chemical and petrochemical plants. It is very difficult to detect points of the pipes where corrosion has considerably developed because the pipes are usually covered with insulations and cannot be inspected directly. Moreover, the length of the pipes is more than 1000km, then it needs huge time and cost to inspect all the deteriorated pipes. Estimation method of inspective pipes based on past case databases has been developed as an inspection support system. The estimation accuracy depends on the number of effective cases recorded in the databases and the soundness of the databases. In this research, three kinds of evaluation indexes of the soundness are proposed.