We propose a mechanism that exhibits the coercivity reduction of Nd-Fe-B magnets. The model employed here is a micromagnetic model involving the anisotropy- and exchange-fields obtained by using first-principles calculations. The uniqueness of this model is that it has a c-plane magnetic anisotropy around its surface, in contrast to the bulk c-axis anisotropy. We find that the local c-plane anisotropy drastically affects the coercivity, resulting in a reduction by half of the bulk anisotropy field Hk . We also find that partial substitution of Dy for Nd around the surface actually recovers the coercivity.