  The lattice QCD studies indicate that the critical temperature $T_c \simeq
260-280$ MeV of the deconfinement phase transition in quenched QCD is
considerably smaller than the lowest-lying glueball mass $m_{\rm G} \simeq
1500-1700$ MeV, i.e., $ T_c \ll m_{\rm G}$. As a consequence of this large
difference, the thermal excitation of the glueball in the confinement phase is
strongly suppressed by the statistical factor as $e^{-m_{\rm G}/T_c} \simeq
0.00207$ even near $T \simeq T_c$. We consider its physical implication, and
argue the abnormal feature of the deconfinement phase transition in quenched
QCD from the statistical viewpoint. To appreciate this, we demonstrate a
statistical argument of the QCD phase transition using the recent lattice QCD
data. From the phenomenological relation among $T_c$ and the glueball mass, the
deconfinement transition is found to take place in quenched QCD before a
reasonable amount of glueballs is thermally excited. In this way, quenched QCD
reveals a question ``what is the trigger of the deconfinement phase transition
?''
