A data compression apparatus has a characterizing point extracting part 1 which extracts data expressing characterizing points included in a plurality of data showing a result of carrying out simulation, quantized data generating part 2 which generates quantized data obtained by quantizing data except for data expressing characterizing points, and file number converting part 3 which converts the same types of quantized data including in the quantized data, into a relating file number. In the case of compressing data, data except for the characterizing points is compressed. If the same quantized data is included at the same address location in the previously-compressed file, the quantized data is replaced with the file number of previously-compression file, thereby compressing data at high efficiency.