In this study, the positioning accuracy of RFIDs used for managing medical instruments in a hospital by means of a wireless LAN is experimentally investigated. After a relationship between the positioning error and building materials or structures is examined, some methods of reducing the error, including a method utilizing a threshold setting technique, the Bayes' theorem and their combination are discussed, especially for electromagnetically degraded or shielded circumstances such as around a CT room. It has been made clear that the threshold setting and the Bayes' theorem methods are effective in reducing the positioning errors and one can obtain better results when both methods are simultaneously employed with the error being about 1 to 2 m.