This study investigated the dissolution characteristics of lagoon sludge in the dissolution-filtration process which dissolve nitrate salts contained in sludge by using water. The dissolved amounts of uranium and nitrate increased rapidly and gradually with dissolution time, respectively. The sludge have to be dissolved within short time to dissolve the smallest uranium. The dissolved amount of uranium, nitrate, and calcium except ammonium increased with the adding ratio of water. Operation condition have to be decided under the condition that uranium is leached minimal and waste water is reduced maximal. Optimum condition was 2.5 weight ratio of [H₂O]/[sludge].