  The experimental value of ${\rm H_{3}S}$ isotope coefficient decreases from
$2.37$ to $0.31$ in the pressure range from $130$ GPa to $200$ GPa. We have
shown that the value of $0.31$ is correctly reproduced in the framework of the
classical Eliashberg approach. On the other hand, the anomalously large value
of the isotope coefficient ($2.37$) may be associated with the strong
renormalization of the normal state by the electron density of states.
