The magnetic properties of rare earth magnet are improved by means of forming a novel structure of the cast alloy used for the production of a rare earth magnet, which contains from 27 to 34% by weight of at least one rare earth element (R) including yttrium, from 07 to 1.4% by weight of boron, and the balance being essentially iron and, occasionally any other transition element, and comprises an R.sub.2 T.sub.14 B phase, an R-rich phase and optionally at least one ternary phase except for the R.sub.2 T.sub.14 B phase and the R-rich phase. The novel structure is that the volume fraction (V) in percentage of said R.sub.2 T.sub.14 B phase and said at least one ternary phase is more than 138-1.6r (with the proviso that r is the content of R), the average grain size of the R.sub.2 T.sub.14 B phases is from 10 to 100 .mu.m and, further, the average spacing between the adjacent R-rich phases is from 3 to 15 .mu.m. The novel structure can be formed by by means of feeding alloy melt onto a rotary casting roll, cooling in a temperature range of from melting point to 1000.degree. C. at a cooling rate of 300.degree. C. per second or more, and further cooling in a temperature range of from 800 to 600.degree. C. at a cooling rate of 1.degree. C./second or less.