Hair can be dyed by applying an oxidizing-agent-free, air-oxidative, one-pack hair dye composition to the hair without mixing the hair dye composition with an oxidizing agent and, after the hair is left over for 5 minutes to 1 hour, rinsing off the hair dye composition with water. The hair dye composition comprises: (a) a water soluble salt of a transition metal: 0.001 to 10 wt. % in terms of an anhydride based on the whole composition, (b) a chelating agent capable of coordinating on said transition metal: 0.001 to 10 wt. % based the whole composition, and (c) a color-developing substance: 0.001 to 10 wt. % based on the whole composition. According to the present invention, hair can be dyed by a convenient procedure without damage. The present invention is also excellent in dyeing properties and color variations.