The present invention addresses the problem of providing a first agent composition for dyeing hair, which exhibits high preparation stability even when being stored for a long period of time. In order to solve this problem, provided is a first agent composition for dyeing hair, characterized by containing: (A) an oxidizing dye to which sulfuric acid is added； (B) a sodium alkylsulfate which has 14 or more carbon atoms； and (C) a higher alcohol, wherein the content of component (A) is 2-10 mass%. Thus, the effect of having high preparation stability can be provided even when the content of the oxidizing dye to which sulfuric acid is added is increased.
