  We examine the Coulomb pseudopotential $\mu^*$ in the McMillan equation
applying to the superconductivity of heavily doped semiconductors. Systematic
calculation using the first-principles calculation suggests that $\mu^*$ should
be considered as a variable quantity depending on carrier density $n$ in
semiconductors, although it is usually considered as a constant about 0.1. To
clarify $n-$dependence of $\mu^*$, we solve the McMillan equation inversely for
$\mu^*$ by combining the result of the first-principles calculation and that of
experiments. It indicates that $\mu^*$ decreases with $n$ and becomes negative
under $n \sim 5 \times 10^{-21}[{\rm cm^{-3}}]$. This reduction is explained by
the effect of plasmon which may play an important role in the superconductivity
of low carrier systems such as heavily doped semiconductors.
