  Black hole-neutron star mergers are among the promising gravitational-wave
sources for ground-based detectors, and gravitational waves from black
hole-neutron star mergers are expected to be detected in the next few years.
Simultaneous detection of electromagnetic counterparts with gravitational-wave
detection provides rich information about the merger events. Among the possible
electromagnetic counterparts from the black hole-neutron star merger, the
emission powered by the decay of radioactive r-process nuclei, so called
kilonova/macronova, is one of the best targets for follow-up observation. We
derive fitting formulas for the mass and the velocity of ejecta from a generic
black hole-neutron star merger based on recently performed numerical relativity
simulations. We combined these fitting formulas with a new semi-analytic model
for a black hole-neutron star kilonova/macronova lightcurve which reproduces
the results of radiation-transfer simulations. Specifically, the semi-analytic
model reproduces the result of each band magnitude obtained by the radiation
transfer simulations within ~1 mag. By using this semi-analytic model, we found
that, at 400 Mpc, the kilonova/macronova is as bright as 22-24 mag for the
cases with a small chirp mass and a high black hole spin, and >28 mag for a
large chirp mass and a low black hole spin. We also apply our model to
GRB130603B as an illustration, and show that a black hole-neutron star merger
with a rapidly spinning black hole and a large neutron star radius is favored.
