A rare earth metal oxysulfide represented by a general formula R.sub.2O.sub.2S (R denotes one kind or two or more kinds of rare earth elements to be selected from a group of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, as wells as Y) is formed into spherical granules. The mean particle size of the granules is 0.05 1 mm and their relative density is 96% or over. The granules are used as a regenerative material at the liquid helium temperature.