  Single-crystal neutron inelastic scattering was performed in order to
investigate the magnetic excitations in the spin gap system TlCuCl$_3$. The
constant-${\bf Q}$ energy scan profiles were collected in the $a^*-c^*$ plane.
Three excitations are observed for $E{\leq}15$ meV. One of the excitations is
identified to be magnetic excitation. The lowest magnetic excitation with
$E\sim 0.5$ meV occurs at ${\bf Q}=(1, 0, 1)$, as observed in KCuCl$_3$. The
dispersion relation of the magnetic excitation can be fitted to the dispersion
formula derived from the weakly coupled dimer model. The intradimer interaction
is evaluated as $J=5.23$ meV, which coincides with the value estimated from the
susceptibility data. However, one of the interdimer interactions obtained is so
large that the weakly coupled dimer model is broken down.
