A thin arc segment magnet made of .[.a.]. .Iadd.an R-T-B based, .Iaddend.rare earth sintered magnet substantially comprising 28-33 weight % of R and 0.8-1.5 weight % of B, the balance being substantially .[.Fe.]. .Iadd.T.Iaddend., wherein R is at least one rare earth element including Y, and T is Fe or Fe and Co, which has an oxygen content of 0.3 weight % or less, a density of 7.56 g/cm.sup.3 or more, a coercivity iHc of 1.1 MA/m (14 kOe) or more at room temperature, and an orientation Br/4.pi.I.sub.max of 96% or more in an anisotropy-providing direction at room temperature can be produced by using a slurry mixture formed by introducing fine alloy powder of the above composition into a mixture liquid comprising 99.7-99.99 parts by weight of a mineral oil, a synthetic oil or a vegetable oil and 0.01-0.3 parts by weight of a nonionic surfactant and/or an anionic surfactant.