PURPOSE: To easily add a new function of a program without changing its basic function.

CONSTITUTION: Each of a host computer 1 and terminals 2-1 to 2-3 has the client programs A to D for the basic functions which show the corresponding characters at their own display parts and also transmit these characters to other devices to display the characters of the same contents when the text data are inputted. When a new function is added, the computer 1 adds a client program E which offers an additional function. When the text data are inputted from another terminal 2-1, for example, the program E decides whether the specific text data that is previously decided is included in the input text data. If the specific data is included, the program E carries out the processing corresponding to the included text data. This processing result is transferred to a client program D as the text data and then transmitted to the terminals 2-1 to 2-3.