To provide a communication system including a mobile terminal which can prevent leak of information even if the mobile terminal is lost and reproduce previously accumulated confidential data without losing newly created and updated confidential data. A mobile terminal includes connection state judging device which detects a line connection state with a server. When the line connection state is a broadband connection, the data is read directly from and written directly into a data storing device of the server. When the line connection state is a narrowband connection, the data is temporarily read out and written into a data storing device of the terminal. When the operation of the mobile terminal is completed or a security operation starting instruction is issued by a user, the updated confidential data is written into the data storing device of the server and then the confidential data is erased from the mobile terminal.