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 $Esim 0.5$ 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.
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