A hair treatment method entailing sequentially (i) preparing a hair cosmetic by mixing a first agent containing an alkali agent and a second agent containing hydrogen peroxide to form a mixed liquid, (ii) discharging the mixed liquid in the form of a foam from a foamer vessel without a propellant, (iii) applying the mixed liquid to hair, (iv) allowing the mixed liquid to stand for 3 to 60 minutes; and (v) rinsing the mixed liquid away.