To obtain an acidic hair dye bearing such characteristics as excellent temperature stability and excellent operability when applied to the hair.

This acidic hair dye, which is a composition comprising an acid dye, dyeing auxiliary and acid, is obtained by the following method: first, a hydrophilic nonionic surfactant of the formula (R is a ≥8C alkene, etc.; X is an ether group, etc.; (n) is an integer of 15-120) such as an ethylene oxide 15-80mol adduct of lanolin, etc., is added to an aqueous solvent followed by addition of an oily phase and a dyeing auxiliary to prepare an oil-in-water- soluble solvent type emulsion; next, water, an acid dye and an acid are added to the above emulsion into an oil-in-water type emulsion.