A data compression apparatus for compressing structured data is provided. The data compression apparatus includes 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, an index data generating unit configured to derive a common vocabulary set from a combination of at least two of the schema files among the input schema files and generate index data for associating an index with each of vocabulary items included in the common vocabulary set, and a replacement unit configured to replace the vocabulary included in the structured data item with a corresponding index on the basis of the index data for each of the structured data items corresponding to the schema files included in the combination of schema files.