1. A data compression apparatus for compressing structured data, comprising: a processor; an input unit configured to input a plurality of structured data items to be compressed and a plurality of schema files each corresponding to one of the structured data items; a similarity deriving unit configured to derive similarity between at least two schema files among the plurality of input schema files; a generating unit configured to use the processor to, in a case where the similarity is equal to or higher than a threshold value, calculate a union of sets of vocabulary items in the at least two schema files and generate one conversion table by associating an index number with each of the vocabulary items from which the union of sets has been calculated, wherein a vocabulary item of the schema file and the index number are associated with each other in the conversion table; and a replacement unit configured to replace a vocabulary item of each of the structured data items corresponding to the schema file with a corresponding index number, by using the conversion table and to obtain compressed structured data.