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 ( \leq 45 ^ { \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 \leq 20 ^ { \circ } , \leq 45 ^ { \circ } , and for \leq 90 ^ { \circ } , respectively . We show that the 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 .