  We study the temperature dependence of the orbital susceptibility of an
excitonic insulator on the basis of a two-band model. It is shown that a
drastic change (an anomalous enhancement) in susceptibility as a function of
temperature occurs owing to the occurrence of additional orbital susceptibility
due to the excitonic gap. We calculate explicitly the temperature dependence of
orbital susceptibility for a model of Ta$_2$NiSe$_5$, and show that the result
is consistent with experimental results.
