1. A computer-implemented method for designing an application program for a data processing operation having a plurality of processes, said method comprising the steps of: editing a data flowchart designating a flow of data for a sequence of said processes in said data processing operation by interactive operation on said computer by an operator, said data including names of said processes, names of data items to be input to said processes and corresponding attributes of said data items; producing a system flowchart on the basis of said data flowchart by the steps of: converting said processes into symbols representing programs for executing said unit functions, converting said data into symbols representing files and forms processed in said programs, and concatenating said programs and said files and forms in a processing sequence; editing program specifications by further interactive operation on said computer by the operator on the basis of said system flowchart, wherein said program specifications indicate a program name, a program pattern name, input file name and output file name; and producing a source program on the basis of a program pattern designated by said program specifications and the attributes corresponding to the program pattern name.