PROBLEM TO BE SOLVED: To provide a hair cosmetic comprising a cuticle cell differentiation promotion active component capable of leading appearance and properties of the hair itself to preferable conditions from a root by improving a function of S100A3 protein being closely related to cuticle cell differentiation process to smoothly promote the cuticle cell differentiation process.SOLUTION: A cuticle cell differentiation promoter includes, as an active component, at least one kind of polyphenol-related compounds selected from esculetin, quercetin and baicalein.