  We study the electron-hole pair (or excitonic) condensation in the extended
Falicov-Kimball model at finite temperatures based on the cluster
mean-field-theory approach, where we make the grand canonical
exact-diagonalization analysis of small clusters using the sine-square
deformation function. We thus calculate the ground-state and finite-temperature
phase diagrams of the model, as well as its optical conductivity and
single-particle spectra, thereby clarifying how the preformed pair states
appear in the strong-coupling regime of excitonic insulators. We compare our
results with experiment on Ta$_2$NiSe$_5$.
