  The atmospheric muon neutrino deficit, which was possible evidence of
$\nu_\mu \leftrightarrow \nu_\tau$ oscillation, was reported by the Kamiokande
experiment from 1988. Many experimental efforts were made to examine the
Kamiokande results. Experiments which contributed to the confirmation of
$\nu_\mu \leftrightarrow \nu_\tau$ oscillation are reviewed. Especially,
long-baseline neutrino-oscillation experiments are described in detail.
