To produce an alloy for an R-T-B-M based sintered magnet which is a sintered magnet having a high residual magnetic flux density and a high coercive force.

In order to produce an R-T-B-M based sintered magnet where a Dy-rich R2T14B exists in the main phase outer shell of crystal grains over the whole sintered magnet, heat treatment is performed continuously or intermittently for 10 minutes or longer but 48 hours or shorter at 600-1000°C under atmospheric pressure of 10 Pa or lower, while moving an R-T-B-M mother alloy 1 and an RH diffusion source 2 of a metal or an alloy of a heavy rare-earth element RH in a processing chamber 3, thus creating a region where the concentration of the heavy rare-earth element RH is high at the interface of the main phase of an alloy for R-T-B-M based sintered magnet, i.e. a crystal of a R2T14B compound, and other phase.