the SSM, the ratio is . The result clearly indicates the deficit of solar neutrinos. Atmospheric neutrino Atmospheric neutrinos are secondary cosmic rays produced by the decay of particles resulting from interactions of primary cosmic rays (mostly protons) with Earth atmosphere. The observed atmospheric neutrino events fall into four categories. Fully contained (FC) events have all their tracks in the inner detector, while partially contained (PC) events have escaping tracks from the inner detector. Upward through-going muons (UTM) are produced in the rock beneath the detector and go through the inner detector. Upward stopping muons (USM) are also produced in the rock beneath the detector, but stop in the inner detector. The number of observed number of neutrinos is predicted uniformly regardless of the zenith angle. However, Super-Kamiokande found that the number of upward going muon neutrinos (generated on the other side of the Earth) is half of the number of downward going muon neutrinos in 1998. This can be explained by the neutrinos changing or oscillating into some other neutrinos that are not detected. This is called neutrino oscillation; this discovery indicates the finite mass of neutrinos and suggests an extension of the Standard Model. Neutrinos oscillate in three flavors, and all neutrinos have their rest mass. Later analysis in 2004 suggested a sinusoidal dependence of the event rate as a function of "Length/Energy", which confirmed the neutrino oscillations. K2K Experiment The K2K experiment was a neutrino experiment from June 1999 to November 2004. This experiment was designed to verify oscillations observed by Super-Kamiokande through muon neutrinos. It gives first positive measurement of neutrino oscillations in conditions that both source and detector are under control. The Super-Kamiokande detector plays an important role in the experiment as the far detector. Later experiment T2K experiment continued as the second generation follow up to the K2K experiment. T2K Experiment T2K (Tokai to Kamioka) experiment is a neutrino experiment collaborated by several countries including Japan, United States and others. The goal of T2K is to gain deeper understanding of parameters of neutrino oscillation. T2K has made a search for oscillations from muon neutrinos to electron neutrinos, and announced the first experimental indications for them in June 2011. The Super-Kamiokande detector plays as the "far detector". The Super-K detector will record the Cherenkov radiation of muons and electrons created by interactions between high energy neutrinos and water. Proton Decay The proton is assumed to