A phase-change optical storage medium includes a recording layer that has an alloy of Ge—Sb—In—Sn, as a main component in the materials that constitute the recording layer. Composition ratios of Ge, Sb, In and Sn in the alloy satisfy ranges 0.08≦a≦0.20, 0.60≦b≦0.80, 0.05≦c≦0.20, and 0.01≦d≦0.15, where “a”, “b”, “c” and “d” are the composition ratios of Ge, Sb, In and Sn, respectively, at a +b+c+d=1. A percentage of the alloy in the materials that constitute the recording layer may be 50% or higher.