Provided is a method for quickly obtaining an intensity value at a desired m/z value in a compressed data obtained by run-length encoding of a mass analysis data. An index is created by pairing either the start position of a section where zero-intensity consecutively occurs two or more times in an array of an original spectrum data, or the start position of a sequence of data having significant intensity values in an array of the original spectrum data, with the corresponding position in an array of a compressed data. This index is stored separate from the compressed data. The creation of the index does not affect the array of the compressed data. Therefore, the data can be decompressed even by a data processing system that does not use the index. The index helps to quickly locate a compressed data corresponding to the desired m/z and obtain the necessary intensity value.