An objective of the present invention is to provide a rare earth metal-based sintered magnet having imparted thereto sufficient corrosion resistance by an oxidative heat treatment, which is resistant even in an environment of fluctuating humidity, while suppressing the deterioration of the magnetic characteristics ascribed to the oxidative heat treatment, and to provide a method for producing the same. As a means of achieving the objective above, the surface-modified rare earth metal-based sintered magnet of the present invention is characterized in that the surface-modified part comprises a surface-modified layer comprising at least three layers formed in this order from the inner side of the magnet, a main layer containing R, Fe, B, and oxygen, an amorphous layer containing at least R, Fe, and oxygen, and an outermost layer containing iron oxide comprising mainly hematite as the constituent, and the method for producing the same is characterized in that it comprises a step of applying a heat treatment to a bulk magnet body in the temperature range of from 200° C. to 600° C., under an atmosphere with oxygen partial pressure in a range of from 1×102 Pa to 1×105 Pa and water vapor partial pressure in a range of from 0.1 Pa to 1000 Pa (exclusive of 1000 Pa).