The Hongjimun Ogansumun (Five-arched floodgates) is composed mainly of biotite granite, pink feldspar granite and granodiorite that are very similar with granitic rocks around the Seoul. Main damage in the floodgates was gray, yellow and predominantly white discoloration on the surface of stone blocks. All floodgates showed more discoloration in the wall than ceiling, and there were growing stalactite on the ceiling. Scientific analyses determined that the white discoloring substrates were mainly calcite. Therefore, conservation treatment was carried out to remove the substrates by using dry cleaning, high pressure cleaning and chemicals. The floodgates have been restored to good state, but it is still significantly necessary to install drainage on the top of the gates.