Structures of neutrino flavor mixing matrix and neutrino oscillations at CHORUS and NOMAD

We study structures of the neutrino flavor mixing matrix focusing on the neutrino oscillations at CHORUS and NOMAD as well as the one at LSND (or KARMEN). We assume two typical neutrino mass hierarchies $m_3\simeq m_2\gg m_1$ and $m_3\gg m_2\gg m_1(or \simeq m_1)$. Taking into account the seesaw mechanism of neutrino masses, reasonable neutrino flavor mixing patterns are discussed. The observation of the neutrino oscillation at CHORUS and NOMAD presents an important constraint for the structure of the neutrino flavor mixing matrix. The atmospheric neutrino anomaly is discussed in relation to the CHORUS and NOMAD experiments.