A modulating light having a wavelength of 400 nm to 600 nm is irradiated to a portion around hair roots at an energy of 0.01 J/cm 2 to 1 J/cm 2 over a time period of not more than 1 ms as a flash light such that a light absorptive component existing in a human body around the hair roots absorbs the light for modulating hair growth without causing substantial adverse effect.