PURPOSE: To produce the stable modified rosin that absorbs almost no oxygen even when it is left for a long time in a pure oxygen atmosphere and that is a polymerized rosin with a specific property heated in the presence of a disproportionating agent.

CONSTITUTION: A polymerized rosin with a ring and ball softening point of at least 90°C (preferably over 130°C) and an abietic acid content of preferably less than 1% is heated preferably at 200-300°C for 1-4 hours is heated in the presence of a disproportionating agent selected from I, noble metals, S, selenium, hydroxyarylsulfide groups, arylthiols, or dithin derivatives to produce the object modified rosin. The oxygen absorbing amount of the modified rosin is 0.4 wt.% or below even when the rosin is left in a pure oxygen atmosphere of 21 kg/cm2 at 25°C for 14 days.