The present invention relates to a digital hearing aid, which models the structures of external ear canals, sizes and shape characteristics of which differ between respective persons, obtains resonance gains generated due to the structural characteristics of the external ear canals, and performs digitization and signal processing to allow the resonance gains to be used as the gain factors of the digital hearing aid, and thus applies the gain factors to digital signal processing units. Further, the present invention proposes a gain obtainment unit capable of taking both resonance gains, generated due to the structural characteristics, and gains, obtained through a hearing test, into account, thus reducing the time required for gain fitting and possible errors, and optimizing the performance of the digital hearing aid for each individual.