  In framework of the chiral perturbation theory we obtain the phenomenological
relations between decay branches of rare radiative kaon to pion and leptons
$K^+ \to \pi^+ l^+ l^-$ and $K^0_S \to \pi^0 l^+ l^-$ and meson form factors.
The comparison of these results with the present day experimental data shows us
that the ChPT relations for a charge kaon can determine meson form factors from
already measured decay rates at high precision level. However, in the case of
the neutral kaon decays $K^0 \to \pi^0 e^+ e^- (\mu^+ \mu^-)$ the formfactor
data are known to a high precision than data on the differential rates of
radiative kaon decay $K^0 \to \pi^0 e^+ e^- (\mu^+ \mu^-)$.
