  We study a consistent explanation for both deficits of solar neutrino and
atmospheric neutrino due to the neutrino oscillation induced by the mixing of
light neutrinos with a heavy right-handed neutrino. We propose such a
phenomenological neutrino mass matrix that realizes this scenario in the simple
three generation left- and right- handed neutrino framework. Although this
model contains only one nonzero mass eigenvalue $\sim$ 10 eV for these light
neutrino states at the first approximation level, it can be expected to explain
consistently both deficiency due to the appropriate higher order corrections. A
suitable hot dark matter candidate is naturally included in it as its own
feature.
