be absolutely stable in the Standard Model. However, the Grand Unified Theories (GUTs) predict that protons can decay into lighter energetic charged particles such as electrons, muons, pions, or others which can be observed. Kamiokande helps to rule out some of these theories. Super-Kamiokande is currently the largest detector for observation of proton decay. Purification Water purification system The 50 kilotons of pure water is continually reprocessed at rate about 30 tons/hour in a closed system since early 2002. Now, raw mine water is recycled through the first step (particle filters and RO) for some time before other processes, which involve expensive expendables, are imposed. Initially, water from the Super-Kamiokande tank is passed through nominal 1 μm mesh filters to remove dust and particles, which reduce the transparency of the water for Cherenkov photons and provide a possible radon source inside the Super-Kamiokande detector. A heat exchanger is used to cool down the water in order to reduce the PMT dark noise level as well as suppress the growth of bacteria. Surviving bacteria are killed by a UV sterilizer stage. A cartridge polisher (CP) eliminates heavy ions, which also reduce water transparency and include radioactive species. The CP module increases the typical resistivity of recirculating water from 11 MΩ cm to 18.24 MΩ cm, approaching chemical limit. Originally, an ion-exchanger (IE) was included in system, but it was removed when IE resin was found to be a significant radon source. The RO step that removes additional particulates, and the introduction of Rn-reduced air into the water that increases radon removal efficiency in the vacuum degasifier (VD) stage which follows were installed in 1999. After that, a VD removes dissolved gases in the water. These gases are dissolved in water with a serious background of events source for solar neutrinos in the MeV energy range and the dissolved oxygen encourages the growth of bacteria. The removal efficiency is about 96%. Then, the ultra filter (UF) is introduced to remove particles whose minimum size corresponds to molecular weight approximately 10,000 (or about 10 nm diameter) thanks to hollow fiber membrane filters. Finally, a membrane degasifier (MD) removes radon dissolved in water, and the measured removal efficiency for radon is about 83%. The concentration of radon gases is miniaturized by realtime detectors. In June 2001, typical radon concentrations in water coming into the purification system from the Super-Kamiokande tank were less than 2