  We report the study on superconducting pairing mechanism of FeSe via the
pair-breaking effects induced by H$^+$-irradiation combined with
low-temperature specific heat measurements. A multi-gap structure with nodes or
gap minima is suggested in a clean FeSe by the specific heat results. The
suppression of critical temperature $T_c$ with increasing the defect density
manifests a two-step behavior. When the increase in the residual resistivity is
small, $\Delta\rho_0$ $<$ $\sim$4.5 $\mu\Omega$cm, $T_c$ is gradually
suppressed with increasing the density of scattering centers, suggesting the
presence of symmetry-unprotected nodes or gap minima. However, for
$\Delta\rho_0$ $>$ $\sim$4.5 $\mu\Omega$cm, $T_c$ is almost independent of the
scattering, which indicates that the nodes or gap minima are lifted and the
order parameter becomes almost isotropic without sign change. Thus, the
superconductivity in FeSe is found to be realized in symmetry-unprotected nodal
or highly anisotropic $s_{++}$ state.
