Hair dye

[Problem] The purpose of the present invention is to provide a semi-permanent hair-dyeing hair dye having a cumulative hair dyeing property in which hair is gradually dyed slowly by repeating use as a temporary coloring product, and to provide a hair dye having a high level of temporary coloring properties, cumulative hair dyeing properties, low irritation, and drying properties. [Solution] The hair dye according to the present invention is characterized by containing: 0.1-3% by mass of at least one type of water-soluble dye; 3-20% by mass of a hair dyeing auxiliary agent; 20-70 mass% of one or more lower alcohols selected from the group consisting of ethanol, propanol, butanol, isopropanol, and isobutanol; 5-70% by mass of water; and one or more types of acids which are represented by formula (I) and which have a parameter relating to the degree of ionization of acids of 0.15 or less. Formula (I): the degree of ionization (one type of acid) of the acid in the nth stage when 10 mmol/L of the acid is added to