PURPOSE: To provide a cosmetic base compound of a specific peptide derivative, having adsorption action to hair and protecting action to hair and skin, etc., as well as excellent surface activity and giving low stimulation.

CONSTITUTION: The objective cosmetic base contains a peptide derivative at one or more cysteine residues of a peptide containing one or more cysteine residues wherein the structure after the thiol group of the side chain of the induced cysteine is expressed by the formula (A is ethylene or propylene; R is 9W31C saturated hydrocarbon group or unsaturated hydrocarbon group). The peptide is a keratin peptide produced by hydrolyzing keratin with an acid, an alkali or a proteinase. The average molecular weight of the peptide is preferably 120W3,000.