We propose the Hyper-Kamiokande ( Hyper-K ) detector as a next generation underground water Cherenkov detector . It will serve as a far detector of a long baseline neutrino oscillation experiment envisioned for the upgraded J-PARC , and as a detector capable of observing – far beyond the sensitivity of the Super-Kamiokande ( Super-K ) detector – proton decays , atmospheric neutrinos , and neutrinos from astronomical origins . The baseline design of Hyper-K is based on the highly successful Super-K , taking full advantage of a well-proven technology . Hyper-K consists of two cylindrical tanks lying side-by-side , the outer dimensions of each tank being 48 ( { W } ) \times 54 ( { H } ) \times 250 ( { L } ) { m } ^ { 3 } . The total ( fiducial ) mass of the detector is 0.99 ( 0.56 ) million metric tons , which is about 20 ( 25 ) times larger than that of Super-K. A proposed location for Hyper-K is about 8 km south of Super-K ( and 295 km away from J-PARC ) at an underground depth of 1,750 meters water equivalent ( m.w.e . ) . The inner detector region of the Hyper-K detector is viewed by 99,000 20-inch PMTs , corresponding to the PMT density of 20 \% photo-cathode coverage ( one half of that of Super-K ) . Hyper-K presents unprecedented potential for precision measurements of neutrino oscillation parameters and discovery reach for CP violation in the lepton sector . With a total exposure of 5 years ( one year being equal to 10 ^ { 7 } sec ) to a 2.5 -degree off-axis neutrino beam produced by the 1.66 MW J-PARC proton synchrotron , it is expected that the CP phase \delta can be determined to better than 18 degrees for all possible values of \delta and CP violation can be established with a statistical significance of 3 \sigma for 74 \% of the \delta parameter space if \sin ^ { 2 } 2 \theta _ { 13 } > 0.03 and the mass hierarchy is known . If \sin ^ { 2 } 2 \theta _ { 13 } is as large as 0.1 the mass hierarchy can be determined with more than 3 \sigma statistical significance for 46 % of the \delta parameter space . In addition , a high statistics data sample of atmospheric neutrinos will allow us to extract the information on the mass hierarchy and the octant of \theta _ { 23 } . With a full 10 year duration of data taking , the significance for the mass hierarchy determination is expected to reach 3 \sigma or greater if \sin ^ { 2 } \theta _ { 23 } > 0.4 . Hyper-K can extend the sensitivity to nucleon decays beyond what was achieved by Super-K by an order of magnitude or more . The sensitivities to the partial lifetime of protons for the decay modes of p \rightarrow e ^ { + } \pi ^ { 0 } and p \rightarrow \bar { \nu } K ^ { + } are expected to exceed 1 \times 10 ^ { 35 } years and 2 \times 10 ^ { 34 } years , respectively . This is the only known , realistic detector option capable of reaching such a sensitivity for the p \rightarrow e ^ { + } \pi ^ { 0 } mode . The scope of studies at Hyper-K also covers high precision measurements of solar neutrinos , observation of both supernova burst neutrinos and supernova relic neutrinos , dark matter searches , and possible detection of solar flare neutrinos . The prospects for neutrino geophysics using Hyper-K are also mentioned .