Alkaline elements in concrete protect reinforcing bars from oxidisation, but carbon dioxide contained in the air neutralises them and gradually neutralisation progresses. When neutralisation reaches the reinforcing bars, rain water which enters through the cracks rusts these bars. Then the bars, expanded by rusting, induce further cracking. The degree of neutralisation depends chiefly upon the depth of concrete cover, but its environment, and the quality of concrete -the more impervious to the air the concrete is, the better-, are also important. According to the brief description of building No. 30 in the history of Mitsubishi Mine Corporation, the depth of concrete cover is only 2 centimeters which is half that of contemporary buildings. In view of this building's inexact execution, the depth of cover could be less than 1 centimeter in parts and our research proved this was in fact the case in those areas where repair with mortar or repair by changing the reinforcing bars and re-covering had not been done. Although it comes as no surprise that there was no standard of covedring depth in the early days of concrete construction, lack of recognition of its importance as well as this island's severe environment caused serious weathering of the early reinforced concrete apartment blocks.