  We report the temperature evolution of the Dirac band in semiconducting
zirconium pentatelluride (ZrTe$_5$) using magneto-infrared spectroscopy. We
find that the band gap is temperature independent at low temperatures and
increases with temperature at elevated temperatures. Although such an
observation seems to support a weak topological insulator phase at all
temperatures and defy the previously reported topological phase transition
(TPT) at an intermediate temperature in ZrTe$_5$, we show that it is also
possible to explain the observation by considering the effect of
conduction-valence band mixing and band inversion with a strong topological
insulator phase at low temperatures. Our work provides an alternative picture
of the band gap evolution across TPT.
