  The integral representation for Isgur -- Wise function (IWF) is obtained in
the framework of instant--form relativistic Hamiltonian dynamics for mesons
with one heavy quark. The upper and lower limits are calculated for the slope
parameter of IWF $\rho^2$ by the model independent way. IWF is calculated for
different wave functions of quarks in the meson. The difference between the
limits of $\rho^2$ equals 1/3. The constraint on the slope parameter is in a
good agreement with experiments. The weak dependence of IWF on the choice of
wave functions is found.
