Provided is a one-part air-oxidative hair dye composition containing (A) 5,6-dihydroxyindoline-2-carboxylic acid or a salt thereof, (B) an alkali agent selected from sodium hydroxide, potassium hydroxide, ammonia, guanidine, basic amino acids and carbonates, and (C) from 10 to 50 wt. % of a water compatible organic solvent, said composition being substantially free of an alkanolamine, and having a pH of from 8 to 12. This one-part air-oxidative hair dye composition has excellent hair dyeing power.