Objects of the invention are to enable different kinds of communication apparatus having interfaces of different input/output methods and different signal voltages to be connected to the same interface; to achieve communication processing appropriate to the automatically detected attribute of the communication apparatus; to explicitly indicate optimum communication means; and to enable communications to be automatically resumed even if the communication apparatus is replaced in the middle of communication. The communication apparatus comprises: means for sending the connected state of the communication apparatus and an attribute of the communication apparatus which connected to information terminal apparatus; a signal line for sending a connection notification to the information terminal apparatus; and a signal input section and a signal output section for transferring control signals and data to and from the information terminal apparatus.