  Considering the five-dimensional warped spacetime AdS_5 with the D3-brane, we
derive a potential in the fifth dimension, according to which ordinary
particles are initially confined on the D3-brane. It is estimated, however,
that the lightest neutrino with mass m_1 is tunneling away into the extra
dimension. Hence there is a possibility that no neutrinos with mass m_1 exist
in cosmicbackground neutrinos, but surviving neutrinos are those with heavier
masses m_2 and m_3. The other possibilities are also discussed.
