  Specific heat measurements have been performed in the coupled dimer system
TlCuCl$_3$, which has a singlet ground state with the excitation gap $\Delta
\simeq 7.5$K. The cusplike anomaly indicative of the 3D magnetic ordering was
clearly observed in magnetic fields higher than the critical field $H_c$
corresponding to the gap $\Delta$. The phase boundary determined by the present
specific heat measurements coincides with that determined by previous
magnetization measurements. The phase boundary can be described by the power
law $[ H_{\rm c}(T)-H_{\rm g} ] \propto T^{\phi}$ with $\phi=2.1(1)$. This
result supports the magnon Bose condensation picture for the field-induced
magnetic ordering in TlCuCl$_3$ [Nikuni {\it et al}., Phys. Rev. Lett. {\bf 84}
(2000) 5868].
