A modem apparatus performs modulation and demodulation between a telephone circuit and a terminal apparatus. The modem apparatus includes a modem circuit, and a codec circuit performing conversion between an analog signal and a digital signal. Capacitors are provided between the modem circuit and the codec circuit, a signal supplied from the terminal apparatus and a signal supplied from the codec circuit being supplied to the modem circuit, and a signal supplied from the telephone circuit and a signal supplied from the modem circuit being supplied to the codec circuit.