1. An online application processing system having a first computer which acts as a host, communication means, and a second computer which includes a keyboard and a display unit capable of displaying graphics, comprising: said first computer having an application program source code, first screen definition information and online application program generating means, said first screen definition information defining an input-output correspondence table for accommodating field names of data fields and display coordinates in an online application processing screen and for at least specifying an input-output distinction distinguishing an input from an output; and said online application program generation means generating, on the basis of said application program source code and said first screen definition information, an online application program that displays a screen on an apparatus connected with said communication means in accordance with said first screen definition information and to perform online application processing in accordance with processing procedures written in said application program source code; wherein said second computer comprises second screen definition information and GUI processing program generating means, said second screen definition information being acquired by copying said first screen definition information, said GUI processing program generating means generating, on the basis of said second screen definition information, a GUI processing program for interactively communicating with a user through a GUI display and communicating with said online application program via said communication means.