  We study long-term evolution of the matter ejected in a black-hole
neutron-star (BH-NS) merger employing the results of a long-term
numerical-relativity simulation and nucleosynthesis calculation, in which both
dynamical and post-merger ejecta formation are consistently followed. In
particular, we employ the results for the merger of a $1.35\,M_\odot$ NS and a
$5.4\,M_\odot$ BH with the dimensionless spin of 0.75. We confirm the finding
in the previous studies that thermal pressure induced by radioactive heating in
the ejecta significantly modifies the morphology of the ejecta. We then compute
the kilonova (KN) light curves employing the ejecta profile obtained by the
long-term evolution. We find that our present BH-NS model results in a KN light
curve that is fainter yet more enduring than that observed in AT2017gfo. This
is due to the fact that the emission is primarily powered by the
lanthanide-rich dynamical ejecta, in which a long photon diffusion time scale
is realized by the large mass and high opacity. While the peak brightness of
the KN emission in both the optical and near-infrared bands is fainter than or
comparable to those of binary NS models, the time-scale maintaining the peak
brightness is much longer in the near-infrared band for the BH-NS KN model. Our
result indicates that a BH-NS merger with massive ejecta can observationally be
identified by the bright and long lasting ($>$two weeks) near-infrared
emission.
