PURPOSE: To easily extract peculiar information which is a recent remarkable subject by comparing information of frequencies in appearance before storage with information of new frequencies in appearance and extracting and outputting document data including a word in the case that the differential value of the frequency in appearance of this word exceeds a threshold.

CONSTITUTION: Contents of a word appearance frequency-document number correspondence file 10 are updated by periodically automatically executing a word appearance frequency change condition detecting program operated by a document data storage retrieval processing part 9, and differences between frequencies in appearance before update and those after update of the word appearance frequency-document number correspondence file 10 are calculated. If the difference exceeds a preliminarily given threshold or the frequency in appearance before update is 0, the pertinent word and related words are obtained from a related word file 12, and a document number corresponding to them is obtained from the word appearance frequency-document number correspondence file 10. Thereafter, corresponding document data is extracted and outputted from a document data file 11.