Provided is a method of automatically fitting a hearing aid. The method includes entering the audiogram of a test subject, defining criterion gains and SSPLs based on installed criterion gains and SSPLs according to the test subject's audiogram, generating sounds from automatic fitting device, measuring the sounds using probe microphone inserted in external earcanal, adjusting the criterion gain and SSPLs of the hearing aids based on differences between output sound amplitude and measured sound amplitude, and saving the changed values to the hearing aids automatically, in a state in which the hearing aid is worn by a test subject.