Three Flavor Neutrino Oscillations and Application to Long Baseline Experiments

Using the result of the three flavor analysis of the old Kamiokande data, the recent Superkamiokande data of atmospheric neutrinos and the CHOOZ reactor data, it is shown that the third mixing angle ${\theta }_{13}$ is small. It is proposed to determine the small value of ${\theta }_{13}$ and the CP violating phase $\delta$ in very long baseline experiments by measuring the appearance probability $P({\nu }_{\mu }\to {\nu }_e)$ and the T violating effect $P({\nu }_e\to {\nu }_{\mu })-P({\nu }_{\mu }\to {\nu }_e)$ which are enhanced by the matter effect of the Earth.