  The muon- and tau-neutrinos with the mass in the keV range, which are allowed
in a low reheating temperature cosmology, can compose the warm dark matter of
the universe. A model of four light neutrinos including the keV scale $\nu_\mu$
and $\nu_\tau$ is studied, which combines the seesaw mechanism and the Abelian
flavor symmetry. The atmospheric neutrino anomaly is due to the
$\nu_\mu-\nu_\tau$ oscillation. The solar neutrino problem is answered by the
oscillation into the light sterile neutrino, where the SMA, LMA, and LOW-QVO
solutions can be accommodated in our scenario.
