[en] Low-energy spin excitations on polycrystalline LaFeAsO_1−xF_x samples have been studied by inelastic neutron scattering. The Q-integrated dynamical spin susceptibility χ⁽ω) at 11 meV decreases with increasing fluorine content x across the phase transition from antiferromagnetic to superconducting phases. For superconducting samples, a peak in χ⁽ω) develops at ω = E_res below T_c, accompanied by its reduction below E_res. Fe 3d orbital contribution on the dynamical spin susceptibility and the electron scattering between Γ and M Fermi surfaces are discussed.