1. An analyzer analyzing a tubular structure of an object to be examined, comprising: a preparing unit configured to prepare a plurality of sets of three-dimensional image data of the same object examined; a structure extracting unit -configured to extract image data indicative of a three-dimensional tubular structure, set by set, from the plurality of sets of three-dimensional image data, thereby a plurality of sets of structure image data being produced; a reference direction specifying unit configured to specify a reference direction to the plurality of sets of three-dimensional image data; a reference point specifying unit configured to specify a reference point to each center line of the tubular structure contained in each of the plurality of sets of structure image data; a stretched image producing unit configured to produce, from each of the plurality of sets of structure image data, data of a stretched image of the tubular structure in each of plural sections which are mutually the same with regard to three-dimensional positions thereof and determined based on the reference direction, thereby a plurality of sets of data of stretched images being produced; and a stretched image displaying unit configured to display the plurality of sets of data of stretched images aligned based on the reference point.