  In high-superconducting transition temperature ($T_{\rm c}$) iron-based
superconductors, interband sign reversal ($s_{\rm \pm}$) and sign preserving
($s_{\rm ++}$) $s$-wave superconducting states have been primarily discussed as
the plausible superconducting mechanism. We study Co impurity scattering
effects on the superconductivity in order to achieve an important clue on the
pairing mechanism using single crystal Fe$_{1-x}$Co$_x$Se and depict a phase
diagram of a FeSe system. Both superconductivity and structural transition /
orbital order are suppressed by the Co replacement on the Fe sites and
disappear above $x$ = 0.036. These correlated suppressions represent a common
background physics behind these physical phenomena in the multiband Fermi
surfaces of FeSe. By comparing experimental data and theories so far proposed,
the suppression of $T_{\rm c}$ against the residual resistivity is shown to be
much weaker than that predicted in the case of a general sign reversal and a
full gap $s_{\pm}$ models. The origin of the superconducting paring in FeSe is
discussed in terms of its multiband electronic structure.
