  The final state interaction of pions in the decay $K^\pm\to
\pi^+\pi^-e^\pm\nu$ allows to obtain the value of the isospin and angular
momentum zero pion-pion scattering length $a_0^0$. To extract this quantity
from experimental data the radiative corrections (RC) have to be taken into
account. Basing on the lowest order results and the factorization hypothesis,
we get the expressions for RC in the leading and next-to leading logarithmical
approximation. It is shown that the decay width dependence on the lepton mass
$m_e$ through the parameter
$\sigma=\frac{\alpha}{2\pi}\br{\ln\frac{M^2}{m_e^2}-1}$ has a standard form of
the Drell-Yan process and is proportional to the Sommerfeld-Sakharov factor.
The numerical estimations are presented.
