A diffusion treatment device includes: a treatment container including a cylindrical main body and first and second lids, the cylindrical main body having a treatment space which is capable of receiving sintered magnet pieces and RH diffusion sources, the first and second lids being capable of hermetically sealing first and second openings, respectively, at opposite ends of the cylindrical main body; a conveyor for conveying the treatment container by a predetermined distance in an x-axis direction while a longitudinal direction of the treatment container is located in a y-axis direction in a rectangular coordinate system xyz; a heating unit including a lower heating section provided under the treatment container and an upper heating section provided above the treatment container, and a first rotating unit for rotating the treatment container around a y-axis while the longitudinal direction of the treatment container is located in the y-axis direction.