  Neutrinos are known to undergo flavor conversion processes among the three
flavors. The fast flavor conversion (FFC) has been the central piece of flavor
conversions taking place in core-collapse supernovae (CCSNe) due to its shorter
timescale to the completion of flavor conversion compared to other types of
flavor conversion. Although the ordinary collisions between neutrinos and
matter were once thought to decohere neutrinos and thus damp flavor
conversions, it was recently realized that they can also induce the flavor
conversion. The linear analysis showed that the so-called collisional flavor
instability or CFI occurs in the absence of FFC. In this paper, we investigate
if CFI takes place in of the post-bounce core of CCSNe, using the results of
spherically symmetric Boltzmann simulations of CCSNe for four progenitor models
with different masses. We also provide a necessary (but not sufficient)
condition of matter properties for the occurrence of CFI in optically thick and
semi-transparent regions; baryon mass density ($\rho$), electron fraction
($Y_e$), and the degeneracy of electron-type neutrinos ($\eta_{\nu_e}$) need to
be $10^{10} {\rm g/cm^3} \lesssim \rho \lesssim 10^{12} {\rm g/cm^3}$,
$Y_e\lesssim 0.4$, and $\eta_{\nu_e} \lesssim 0.5$, respectively. This
condition allows us to easily locate the place of possible CFI occurence
without detailed stability analyses, which is useful for analyzing CFI in CCSN
models phenomenologically
