An emulsion for hair treatment is provided which is characterized by the following: the formulation (I) is composed of a W/O type emulsion, which allows the main agent to be stable, and contains preferably an oil soluble polymer compound for the emulsifier; the formulation (I), upon being mixed with formulation (II), can easily undergo demulsification and a phase inversion into an O/W type emulsion; and the emulsion has adequate viscosity to prevent dripping from the hair and a superior dyeing capability. Also, 2-formulation type oxidation hair dye with an excellent hair dyeing effect is provided by adding isostearyl alcohol.