A method for producing a sintered R-T-B based magnet includes the steps of: providing a sintered R-T-B based magnet body 1; providing an RH diffusion source including a heavy rare-earth element RH (which is at least one of Dy and Tb) and 30 mass % to 80 mass % of Fe; loading the sintered R-T-B based magnet body 1 and the RH diffusion source 2 into a processing chamber 3 so that the magnet body 1 and the diffusion source 2 are movable relative to each other and are readily brought close to, or in contact with, each other; and performing an RH diffusion process in which the sintered magnet body 1 and the RH diffusion source 2 are heated to a processing temperature of more than 850° C. through 1000° C. while being moved either continuously or discontinuously in the processing chamber.