To provide a method for structure-estimating a compound noun containing unknown words by collecting keys which actually appear in the other places in a document and combining them.

Words obtained by morpheme-analyzing a compound word and which cooccur with a core word are retrieved by using pattern matchers. A cooccurrence example is stored in a data storage area. When the unknown words are detected, they are stored in an unknown storage area (1006 and 1007). A cooccurrence pattern is retrieved on the detected unknown words and a result is added in a cooccurrence data storage area (1008 and 1009). Data on a modification likelihood storage area is constituted from a cooccurrence data base (1010). When the unknown word is detected, a morpheme analysis result is corrected (1011) and a modification relation between the corrected word groups is estimated by using data of the modification likelihood storage area (1012).