A slave computer reads a plurality of input files that have different formats and generates, for each of the input files, an intermediate file that has added thereto, as a join key, data in a type of column that is common to the input files. The slave computer sorts data in each intermediate file in accordance with the join key. The slave computer extracts, from each intermediate file that has data sorted therein, data that has the format of each of the input files so as to generate a plurality of output files that have data sorted therein with respect to each of the input files. The slave computer inputs the plurality of output files generated to a join program that joins data.