PURPOSE: To simplify the program system of a window system by parsing an windowing instruction expressed in natural language to be converted into an intermediate format independent of the specific programming language of an windowing system.

CONSTITUTION: A data processor is provided with a system unit 12, a display 14, a keyboard 16 and a storage device 18 all of which are mutually connected. Then an instruction of opening the window of natural language is inputted by operating the keyboard 16, this instruction is parsed to prepare a case frame for generally specifying different window function so as not to correspond to a code individually used by a (windowing) system 22 to opening a window. Then this case frame is translated to a windowing source code expressed in an individual programming language with respect to the windowing system 22, thereby simplifying the program developing processing of the window system.