5. The apparatus according to claim 4 , wherein: the second acquisition unit is operable to obtain, as the search results, a first value indicating a number of pieces of document data that include the plurality of constituent words in the first document data group and a second value indicating a number of pieces of document data that include the plurality of constituent words in the second document data group, and the generation unit is operable to generate the index on the basis of a result of comparing the first value with the second value.
7. The apparatus according to claim 5 , wherein: the second acquisition unit is further operable to obtain a third value indicating a number of pieces of document data that include the compound word, the number being obtained by searching the first document data group for the pieces of document data, and the generation unit is operable to generate the index further on the basis of a result of comparing the third value with a third threshold value.