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 $f=u,d,\nu$ 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 $\nu_\mu$-$\nu_\tau$ mixing, i.e., $\sin^2 2\theta_{23}\sim 1$, with $m_{\nu 1} \ll m_{\nu 2} \sim m_{\nu 3}$, which has been suggested by the atmospheric neutrino data.