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Neutrino Mixing in a Democratic-Seesaw-Mass-Matrix Model

On the basis of a seesaw-type mass matrix model for quarks and leptons, $M_f \simeq m_L M_F^{-1} m_R$, where $m_L\propto m_R$ are universal for $f=u,d,ν$ and $e$ (up-quark-, down-quark-, neutrino- and charged lepton-sectors), and $M_F$ has a form [(unit matrix)+(democratic-type matrix)], neutrino masses and mixings are investigated. It is tried to understand a large $ν_μ$-$ν_τ$ mixing, i.e., $\sin^2 2θ_{23}\sim 1$, with $m_{ν1} \ll m_{ν2} \sim m_{ν3}$, which has been suggested by the atmospheric neutrino data.