A method for producing a heavy rare earth grain-boundary-diffused RE-Fe-B-based rare earth magnet, the method comprising: step S1 of processing a rare earth magnet sintered body having a composition of RE-Fe-TM-B, wherein RE = rare earth element, Fe = iron, TM = 3d transition metal, and B = boron, and cleaning the processed sintered body by degreasing, pickling and solvent cleaning; step S2 of applying, to the surface of the cleaned sintered body of step S1, an application material; and step S3 of placing the applied sintered body of step S2 in a heating furnace, and diffusing the heavy rare earth into the grain boundaries of the sintered body at a temperature of 600 to 1000°C in a vacuum or inert gas atmosphere, thereby obtaining a diffused material wherein the rare earth magnet sintered body has a composition comprising 27 to 36 wt% of RE, 64 to 73 wt% of Fe, 0 to 5 wt% of TM, and more than 0 wt% and not more than 2 wt% of B; wherein the application material used in step S2 is a first heavy rare earth compound obtained by mixing 10wt% to 25wt% of a Dy-H compound with the balance of a Dy-F compound, or a second heavy rare earth compound obtained by mixing 10wt% to 25wt% of a Tb-H compound with a balance of a Tb-F compound, or a mixture obtained by mixing the first heavy rare earth compound and the second heavy rare earth compound, wherein step S3 further comprises, after the diffusion, first heat treatment at a temperature between 900°C and 1,000°C, followed by second heat treatment at a temperature between 600°C and lower than 800°C, and then third heat treatment at a temperature between 450°C and lower than 600°C, wherein the second heat treatment is performed by rapid cooling at a cooling rate of 90 to 100°C/min and then carrying out heat treatment to a second heat treatment temperature after the first heat treatment, wherein the sintered body used in step S1 is produced by using a magnetic powder having an average particle diameter of 20 to 35 μm and a dispersion coefficient to particle diameter of 25 to 40% as calculated according to the following equation 1, particle diameter dispersion coefficient % of magnetic powder = particle diameter standard deviation μm × 100 / average particle diameter μm of magnetic powder. step S1 of processing a rare earth magnet sintered body having a composition of RE-Fe-TM-B, wherein RE = rare earth element, Fe = iron, TM = 3d transition metal, and B = boron, and cleaning the processed sintered body by degreasing, pickling and solvent cleaning; step S2 of applying, to the surface of the cleaned sintered body of step S1, an application material; and step S3 of placing the applied sintered body of step S2 in a heating furnace, and diffusing the heavy rare earth into the grain boundaries of the sintered body at a temperature of 600 to 1000°C in a vacuum or inert gas atmosphere, thereby obtaining a diffused material wherein the rare earth magnet sintered body has a composition comprising 27 to 36 wt% of RE, 64 to 73 wt% of Fe, 0 to 5 wt% of TM, and more than 0 wt% and not more than 2 wt% of B; wherein the application material used in step S2 is a first heavy rare earth compound obtained by mixing 10wt% to 25wt% of a Dy-H compound with the balance of a Dy-F compound, or a second heavy rare earth compound obtained by mixing 10wt% to 25wt% of a Tb-H compound with a balance of a Tb-F compound, or a mixture obtained by mixing the first heavy rare earth compound and the second heavy rare earth compound, wherein step S3 further comprises, after the diffusion, first heat treatment at a temperature between 900°C and 1,000°C, followed by second heat treatment at a temperature between 600°C and lower than 800°C, and then third heat treatment at a temperature between 450°C and lower than 600°C, wherein the second heat treatment is performed by rapid cooling at a cooling rate of 90 to 100°C/min and then carrying out heat treatment to a second heat treatment temperature after the first heat treatment, wherein the sintered body used in step S1 is produced by using a magnetic powder having an average particle diameter of 20 to 35 μm and a dispersion coefficient to particle diameter of 25 to 40% as calculated according to the following equation 1, particle diameter dispersion coefficient % of magnetic powder = particle diameter standard deviation μm × 100 / average particle diameter μm of magnetic powder.