  We report theoretical and experimental studies of the effect of Zn-impurity
in Fe-based superconductors. Zn-impurity is expected to severely suppress sign
reversed s$_\pm$ wave pairing. The experimentally observed suppression of T$_c$
under Zn-doping strongly depends on the materials and the charge carrier
contents, which suggests competition of $s_{++}$ and $s_{\pm}$ pairings in
Fe-base superconductors. We study a model incorporating both $s_{++}$ and
$s_{\pm}$ pairing couplings by using Bogoliubov de-Gennes equation, and show
that the Zn-impurity strongly suppresses $s_{\pm}$ pairing and may induce a
transition from $s_{\pm}$ to $s_{++}$-wave. Our theory is consistent with
various experiments on the impurity effect. We present new experimental data on
the Zn-doping SmFe$_{1-x}$Zn$_x$AsO$_{0.9}$F$_{0.1}$ of T$_c=$ 50K, in further
support of our proposal.
