PURPOSE: To decrease the physical input actions to a sort data input file by utilizing a fact that plural source data having approximate keys exist within a block of the file and also applying a key sort method.

CONSTITUTION: An input means (I) 11 inputs the sorting interim results through a sorting interim result file 1 for each block. An input means (II) inputs the blocks containing source data based on the address information on a sort data input file 2. A judging means 22 judges whether or not the corresponding source data exists within a block of the file 1. If so, a shift means 31 shifts the source data to write it to a sorting result output file 3. A control means 32 sets the shift conditions to the intermediate data corresponding to the source data shifted to a file 3 and controls said shift conditions. A writing means 33 writes the source data for each block when the file 3 is filled with source data.