  We derive the upper limit to the ejecta mass of S190814bv, a black
hole-neutron star merger candidate, through the radiative transfer simulations
for kilonovae with the realistic ejecta density profile as well as the detailed
opacity and heating rate models. The limits to the ejecta mass strongly depend
on the viewing angle. For the face-on observations ($\le45^\circ$), the total
ejecta mass should be smaller than $0.1\,M_\odot$ for the average distance of
S190814bv ($D=267$ Mpc), while larger mass is allowed for the edge-on
observations. We also derive the conservative upper limits of the dynamical
ejecta mass to be $0.02\,M_\odot$, $0.03\,M_\odot$, and $0.05\,M_\odot$ for the
viewing angle $\le 20^\circ$, $\le 50^\circ$, and for $\le 90^\circ$,
respectively. We show that the {\it iz}-band observation deeper than $22$ mag
within 2 d after the GW trigger is crucial to detect the kilonova with the
total ejecta mass of $0.06\,M_\odot$ at the distance of $D=300$ Mpc. We also
show that a strong constraint on the NS mass-radius relation can be obtained if
the future observations put the upper limit of $0.03\,M_\odot$ to the dynamical
ejecta mass for a BH-NS event with the chirp mass smaller than $\lesssim
3\,M_\odot$ and effective spin larger than $\gtrsim 0.5$.
