To provide a hair-strengthening agent that gives sufficient hair-strengthening effects with good persistency of the effects to a damaged hair, a hair cosmetic composition and a hair-strengthening method using these.

The hair-strengthening agent comprises ectoine as an effective ingredient. The hair cosmetic composition comprises the hair-strengthening agent. The hair-strengthening method comprises applying the composition to the hair. More preferably, the hair cosmetic composition further comprises a nitrogen-containing polymer such as a cationic cellulose derivative, a dimethyldiallyl quaternary ammonium salt derivative, an amino-modified silicone, a polypeptide or the like.