An Internet telephone system is provided with an Internet terminal device capable of connecting with the Internet and transmitting and receiving audio signals for a voice communication. The Internet terminal has a communication unit through for transmitting/receiving the audio signals to/from the Internet. The Internet terminal device has a first interface for outputting/inputting the audio signals for the voice communication and a multifunction device having a telephone function and an Internet telephone function. The multifunction device has a sound input/output unit, a second interface that interfaces between the sound input/output unit and the first interface, a third interface that interfaces between the sound input/output unit and a public telephone network, and a switching system that switches the second interface and the third interface depending on whether the telephone function is used or the Internet telephone function is used.