Authentication information transmission device using inaudible sound wave communication

An authentication method for an authentication system including an authentication device, a user terminal device, and a management server, the authentication method comprising:

• selecting, by the authentication device, a preamble signal, converting authentication information into a sound wave signal by using a conversion method corresponding to the selected preamble signal, and transmitting the sound wave signal to the user terminal device;
• transmitting, by the user terminal device receiving the sound wave signal, an authentication information retransmission request signal to the authentication device;
• selecting, by the authentication device, a preamble signal corresponding to a next turn subsequent to a preamble signal previously selected in a pre-stored preamble signal selection order, converting the authentication information into a sound wave signal by using a conversion method corresponding to the selected preamble signal, and retransmitting the sound wave signal to the user terminal device;
• determining, by the user terminal device, whether or not the preamble signal included in the sound wave signal is a preamble signal in the pre-stored preamble signal selection order;
• and
• transmitting, by the user terminal device, the authentication information retransmission request signal to the authentication device when a determination is made that a preamble signal not in the selection order is included in the sound wave signal, and converting the authentication information in the form of a sound wave signal into digital information, and transmitting the authentication information to the management server when a determination is made that the preamble signal in the selection order is included.