PURPOSE: To efficiently manufacture a denatured rubber excellent in green strength in uncured state, posture fatigue resistance, resiliency, etc., useful for tires, etc., by the reaction, in the presence of a Lewis acid, of glyoxylic anhydride with unsaturated carbon bond-carrying rubber.

CONSTITUTION: An unsaturated carbon bond-carrying rubber (e.g. natural rubber, guayule rubber, polyisoprene rubber, styrene-butadiene rubber) is made to react, in the presence of a Lewis acid [e.g. SnCl4, BF3O(C2H5)2, TiCl4], with pref. 0.01W5pts.wt., per 100pts.wt. of said rubber, of glyoxylic anhydride, pref. at -10W60°C for 10secW10hr, thus performing the objective rubber modification. The amount of said Lewis acid to be used is pref. 0.05W2mol per mol of the glyoxylic anhydride.