The uranium enrichment demonstration plant (UEDP) in Ningyo-toge was used to demonstrate uranium enrichment by the gas centrifuge (GCF) method, and was operating continuously from 1988 to 2001. As a result, significant uranium was deposited in the equipment mainly as intermediate uranium fluorides. System chemical decontamination using IF7 gas was proposed as an efficient decontamination method. The secondary waste characteristic of IF7 treatment is IF5 and minor adsorbent. In addition, IF5 is easy to convert to IF7 and re-use for system decontamination. The IF7 treatment technique is performed at room temperature and very low pres­sures such as 10-45 hPa. Secondary reaction is insignificant in IF7 treatment except for the reaction between IF7 gas and the intermediate uranium fluo­ride. The weights of uranium deposited in the cascades were approximately 700 kgU per cascade before IF7 treatment. The IF7 treatment period for each cascade is 60 days applying the near-optimal processing conditions. More than 96% of uranium was recovered from the cascade system. As a result, the U radioactivity of the main parts of the GCF fell to 1.0 Bq/g and below.