A unique comment keyword is given to a comment statement in a source code. If the comment statements are insufficient, a comment including the comment keyword is inserted in this insufficient part thereof. After this processing, the comment statements are extracted from the source code to create specification data. The specification data is displayed on a display 15. An operator completes the specification data by editing. The comment statement in the source code is replaced with the comment statement in the complete specification data based on the comment keyword being used as a key.