  The neutron-skin thickness of asymmetric semi-infinite nuclear matter is
shown to be a function of Coulomb energy, the asymmetry-energy
coefficient($J$), the slope($L$) of the asymmetry energy, and the
incompressibility coefficient, in addition to he Fermi momentum and the
asymmetry parameter. The relational formula is derived on the basis of the
Hugenholtz-Van Hove theorem in the mean-field approximation for nuclear matter.
Using the formula as a guide, the neutron-skin thickness($\delta R$) in
$^{208}$Pb is examined. The linear correlation between $L$ and $\delta R$
appears as a kind of spurious ones through the model-dependent correlation of
$L$ with $J$ which is included in the main components of the formula.
