PURPOSE: To dispersea polymer containing polar groups uniformly in a rubber, by mixing the polymer with a solution or latex of a diene type rubber in an emulsion or solution state, and adding a specific ionic dissociable sulfate to the resultant mixture.

CONSTITUTION: A polymer containing polar groups is mixed with a solution or latex of a diene type rubber in an emulsion or solution state, and an ionic dissociable sulfate expressed by the formula Mx(SO4)y (M is a metal of GroupsIW VIII in the periodic table; x and y are integers corresponding to the valence) is then added to the resultant mixture. A natural polymer, e.g. protein or lignin, or a synthetic polymer, e.g. a polymer having coordinating power and hydroxyl, thiohydroxy, carbonyl, carboxyl, amino or amide groups, is used as the polymer having the polar groups.