  We propose a scenario that the mass splitting between the first generation of
the heavy Majorana neutrino and the other two generations of degenerate heavy
neutrinos in the seesaw framework is responsible for the deviation of the solar
mixing angle from the maximal mixing, while keeping the maximal mixing between
the tau and muon neutrinos as it is. On top of the scenario, we show that the
tiny breaking of the degeneracy of the two heavy Majorana neutrinos leads to
the non-zero small mixing angle $U_{e3}$ in the PMNS matrix and the little
deviation of the atmospheric neutrino mixing angle from the maximal mixing.
