Natural Neutrino Mass Matrix

Naturalness of the neutrino mass hierarchy and mixings is studied. We select a few neutrino mixing patterns, which could lead the natural neutrino mass matrix among ten mixing patterns. Furtheremore, some mixing patterns with three degenerate mass solution lead to the unnatural right-handed Majorana mass matrix in the see-saw mechanism if the natural Dirac neutrino mass matrix is taken as the natural one in the quark sector. It is shown that only two mixing patterns without the large mixing lead to the natural right-handed Majorana mass matrix although there should be the huge mass hierarchy such as ${\cal O}(10^6)$ in order to reproduce the inverse mass hierarchy of the light neutrinos.