To obtain a humidity hardenable graft copolymer useful for one- component type coating material and sealing agents, which does not give a problem of toxicity or stimulation, shows excellent drying ability and does not damage film properties, and to provide a coating composition containing the copolymer.

The graft copolymer is the one (I) obtained by copolymerizing 1 to 30 wt.% of a cellulose ester derivative (a) obtained by ring-opening graft polymerization of a cyclic ester on a cellulose ester having a hydroxyl group with 70 to 99 wt.% of a polymerizable unsaturated monomer (b) (wherein the sum of (a) and (b) is 100 wt.%), and the introduction of an isocyanate group component to the graft copolymer is conducted by using an isocyanate group- containing unsaturated monomer (f) as a part of the polymerizable unsaturated monomer (b), the content of the isocyanate group being 0.5 to 28.5 wt.% of the graft copolymer (I).