A communications system includes a device connected to information terminals to enable a plurality of information terminals, mutually exchange data via a global network and enable highly confidential mutual communications between the information terminals included. The communications device includes a unit storing user authentication information for performing user authentication via the connected information terminal and pre-storing a device authentication listing pieces of device information in authentication of each communications device with regard to all the communications devices in the same group, the device authentication list being pre-stored in a state where the device authentication list is inaccessible from the user. When exchange of data is performed between the information terminals via the global network, the communications device configured to carry out user authentication process with the information terminal using the user authentication information and device-to-device authentication process with another communications device by referring to the device authentication list.