The corrugated cement slate board which includes asbestos fiber had been generally used as wall and roof members for many plant buildings, because it has high fire and water resistance. From this fact, it is thought that a huge number of same type slate boards still continue to use throughout Japan. On the other hand, the accident reports for deteriorated slate board are increasing such as flying apart by collision when strong wind and breaking by workerâs weight. Therefore, it will be required technical diagnosis based on remaining thickness for drawing reasonable maintenance plan of aged slate boards. In this study, the bending test and impact test are carried out for clarifying between the mechanical properties and remaining thickness for 16+12 corrugated cement slate boards which were dismantled from plant buildings at coastal area in Seto Inland Sea. These slate boards are classified large or small wave shapes and had been used for over 30 years. The remaining bending strength and stiffness of these slate boards were compared with current JIS standards. From the experimental results, the main conclusions are as follows; 1) It will be thought that if the average thickness was decreased more than 10% comparing manufactured initial thickness (6.3mm), the remaining bending strength may not be able to satisfy current JIS standards. In this case, the severe thorough-hole may be caused by any external impactive force. 2) The bending stiffness of deteriorated slate boards will depend on usage environment because can be grouped by sampled plant buildings.