  The modification of the naive potential model of the heavy meson is
considered on the basis of the single-time prescription for the quark-meson
form factor and the covariant choice of the quatk relative momentum. The
application of the model to the description of the semileptonic decays of the
beauty meson allows one to determine the Isgur-Wise function slope
$\rho^2\simeq 1.25$ and to extract the value $|V_{bc}|= 0.038\pm 0.003$ from
the experimental data.
