  We study the vicinal surface with adsorption below the roughening
temperature, using the restricted solid-on-solid model coupled with the Ising
model. We calculate the step tension $\gamma$ and the step-interaction
coefficient $B$ by employing a variant of the density matrix algorithm. We find
a ``stiffening transition'' at a temperature $T_{\rm s}$ where $B$ vanishes. At
$T_{\rm s}$ surface free energy has the form $f(p)- f(0)= \gamma p +
({const.})p^5 + ...$ ($p$: surface gradient), which differs from the well-known
$p$-$p^3$ form.
