An R—Fe—B based thin film magnet including an R—Fe—B based alloy which contains 28 to 45 percent by mass of R element (where R represents at least one type of rare-earth lanthanide elements) and which is physically formed into a film, wherein the R—Fe—B based alloy has a composite texture composed of R2Fe14B crystals having a crystal grain diameter of 0.5 to 30 μm and R-element-rich grain boundary phases present at boundaries between the crystals. The magnetization characteristics of the thin film magnet are improved. The R—Fe—B based thin film magnet can be prepared by heating to 700° C. to 1,200° C. during physical film formation or/and the following heat treatment, so as to grow crystal grains and form R-element-rich grain boundary phases.