PURPOSE: To prevent a rubber from early curing without bloom development on the cured rubber surface, by incorporation of a specific phenylenediamine compound in carrying out a thiuram-nonsulfur cure of a diene-based unsaturated rubber.

CONSTITUTION: The objective early cure prevention for rubber can be accomplished by incorporating, in carrying out a thiuram-nonsulfur cure of a diene- based unsaturated rubber, 0.1W5(pref. 0.1W3)pts.wt., based on 100pts.wt. of said rubber, of at least one kind of phenylenediamine compound of formula II (R is H, alkyl or phenyl; n is 0W4, X is S, O or NH, Y is phenylene) prepared by adding formation, while stirring on heating to a mixture of a compound of formula I (R' is halogen, alkyl, alkoxy, acetamide, nitro, or amino), p- phenylenediamine and ethanol to make a reaction.