A polymer treatment agent for rehabilitating damaged hair contains a block copolymer including a hydrophilic polymer chain segment and a hydrophobic polymer chain segment that is derived from a polyamino acid. The polymer treatment agent is not limited to methods for rehabilitating damaged head hair, but also can be applied to body hair including eyelashes and eyebrows； furthermore, it can increase the physical strength of hair even when applied in a very small amount and without using a silicone component.
