A method of producing a sintered R-T-B based magnet includes providing a sintered R-T-B based magnet work, an RH compound (at least one selected from RH fluorides, RH oxides, and RH oxyfluorides), and an RL-Ga alloy, where the sintered magnet work contains R: 27.5 to 35.0 mass %, B: 0.80 to 0.99 mass %, Ga: 0 to 0.8 mass %, M: 0 to 2 mass % (where M is at least one of Cu, Al, Nb and Zr), and T: 60 mass % or more; a diffusion step of, while keeping the RH compound and the RL-Ga alloy in contact with a surface of the sintered magnet work, performing a first heat treatment between 700° C. and 950° C. to increase the RH amount contained in the sintered magnet work by between 0.05 mass % and 0.40 mass %; and performing a second heat treatment between 450° C. and 750° C. but which is lower than the first heat treatment.