A calling side ISDN terminal adapter (TA) judges if Internet call is possible or not when the telephone number of a destination side telephone set is specified by a calling side telephone set. If judged possible, a predetermined number, and ciphered Internet connection member identification number and password are set on the sub-address of call setting, and sent out to a destination side ISDN TA. When receiving the predetermined number, the destination side ISDN TA rings the bell of the destination side telephone set, and when it is off the hook, it connects to the destination side access point by the Internet connection member identification number and password, and notifies the calling side ISDN TA by putting the destination side IP address acquired at the destination side access point on the sub-address of call setting.