To realize a method and an apparatus for effectively compressing a collection of data like X-ray detection data in an X-ray CT apparatus.

In order to compress the data group formed by successive generation of a plurality of data groups each group (j) of which has the predetermined number of data arrayed in a predetermined order, data in each group except the first group is represented by a difference from data in the same place (i) in the just preceding group. Original data are adopted when the difference exceeds a predetermined threshold, and information indicating the start of a difference data string is added before a string of data represented by differences, and information indicating the end of the difference data string is added before original data following data represented by differences.