A method for producing a sintered magnet, the method comprising the steps of: providing a sintered R-T-B based magnet body, of which the R fraction that is defined by the content of a rare-earth element falls within the range of 31 mass% to 37 mass%; providing an RH diffusion source including a heavy rare-earth element RH, which is at least one of Dy and Tb, and 40 mass% to 80 mass% of Fe; loading the sintered magnet body and the RH diffusion source into a processing chamber so that the magnet body and the diffusion source are movable relative to each other and readily brought close to, or in contact with, each other; and performing an RH diffusion process by conducting a heat treatment on the sintered magnet body and the RH diffusion source at a process temperature of 700 °C to 1000 °C while moving the sintered magnet body and the RH diffusion source either continuously or discontinuously in the processing chamber, wherein the moving includes the step of rotating or rocking the processing chamber, or subjecting the processing chamber to externally applied vibrations, or providing a stirring means in the processing chamber, without making the sintered R-T-B based magnet body chip or fracture, wherein the sintered magnet body has an effective rare-earth content of 28 mass% to 35 mass%, wherein the effective rare-earth content is calculated by the following equation: effective rare-earth content = (R fraction mass %) - (6xO fraction mass %) + (8xC fraction mass %) + (10xN fraction mass %), wherein the mass % contents are based on the composition of the sintered magnet body, wherein the O, C and N mol fractions in the sintered R-T-B based magnet body are controlled to fall with the ranges of 0.05 to 0.5 mass%, 0.01 to 0.1 mass% and 0.01 to 0.1 mass%, respectively, and wherein the volume percentage of an RHFe 2 compound such as DyFe 2 and/or an RHFe 3 compound such as DyFe 3, included in the RH diffusion source, becomes 90% or more. providing a sintered R-T-B based magnet body, of which the R fraction that is defined by the content of a rare-earth element falls within the range of 31 mass% to 37 mass%; providing an RH diffusion source including a heavy rare-earth element RH, which is at least one of Dy and Tb, and 40 mass% to 80 mass% of Fe; loading the sintered magnet body and the RH diffusion source into a processing chamber so that the magnet body and the diffusion source are movable relative to each other and readily brought close to, or in contact with, each other; and performing an RH diffusion process by conducting a heat treatment on the sintered magnet body and the RH diffusion source at a process temperature of 700 °C to 1000 °C while moving the sintered magnet body and the RH diffusion source either continuously or discontinuously in the processing chamber, wherein the moving includes the step of rotating or rocking the processing chamber, or subjecting the processing chamber to externally applied vibrations, or providing a stirring means in the processing chamber, without making the sintered R-T-B based magnet body chip or fracture, wherein the sintered magnet body has an effective rare-earth content of 28 mass% to 35 mass%, wherein the effective rare-earth content is calculated by the following equation: effective rare-earth content = (R fraction mass %) - (6xO fraction mass %) + (8xC fraction mass %) + (10xN fraction mass %), wherein the mass % contents are based on the composition of the sintered magnet body, wherein the O, C and N mol fractions in the sintered R-T-B based magnet body are controlled to fall with the ranges of 0.05 to 0.5 mass%, 0.01 to 0.1 mass% and 0.01 to 0.1 mass%, respectively, and wherein the volume percentage of an RHFe 2 compound such as DyFe 2 and/or an RHFe 3 compound such as DyFe 3, included in the RH diffusion source, becomes 90% or more.