Recently, neutrino oscillations due to mixing of mass eigenstates have been discovered in atmospheric and solar neutrino experiments such as Super-K and SNO as well as in KamLAND and K2K experiments using prepared neutrino sources. In the mixing matrix of three neutrino generations, two parameters have yet to be determined: the smallest mixing angle, θ₁₃, and the CP violating phase, δ_CP . Knowing the size of θ₁₃ will define the future direction of investigating neutrino oscillation.