A word id extracted from dated document data, and the number of occurrences of
the word can be obtained by adding up the number of the extracted word for each
field and period. Furthermore, a word indicating a large number of occurrences
in each field and period is extracted as a characteristic word. When a user specifies
a field and a period, characteristic words in the document data in the specified
period are displayed. When the user selects a specific characteristic word, the
header, etc. of the document data containing the characteristic word is displayed.
