To customize an item name and an I/O document name included in a message text and a help text by utilizing customizing information originally prepared for the construction of a program source and a data base.

The changed information on an item and a module changed by customizing a business processing program is previously stored in a customizing information data base 2 and a substitution reserving character string and a customizing information retrieving key corresponding to the character string are previously described in a message text and a help text 5. A customizing source generation processing part 6 substitutes the changed information for a character string in a message file and a help file and outputs the substituted files as new files 7, 8.