A content sharing system in which a plurality of terminal devices that enable voice calls and data communication to each other, switching equipment that mediates the voice calls, and a content sharing server that mediates the data communication are connected to each other, each mobile device comprising: a voice call module, a content sharing application that generates incremental update commands in response to one user operation and sends the incremental update commands to another mobile device with respect to the same content; first and second communication control units that perform data communication according to first and second communication protocols; and a protocol selection unit that sends a desired protocol to the content sharing server according to the status of the terminal device, receives the determined protocol and switches the communication protocol to this protocol.