1. An X-ray diagnosis apparatus, comprising: an X-ray tube configured to generate X-rays; an X-ray collimator configured to variably narrow the X-rays from the X-ray tube; an X-ray detector configured to detect X-rays transmitted through a subject; a rotating mechanism configured to rotate the X-ray tube around the subject together with the X-ray detector; an image reconstruction unit configured to reconstruct a three dimensional image based on a plurality of image data in different radiographing directions in which radiography is repeated while the X-ray tube and the X-ray detector rotate around the subject; a region-of-interest setting unit configured, for each direction of the different radiographing directions, to obtain a moving image of an area of movement of the subject radiographed from the direction, the moving image including a sequence of frames, to obtain a first feature region in a first frame of the sequence, to search each subsequent frame in the sequence to identify corresponding feature regions most similar to the first feature region so as to identify a sequence of feature regions, and to set a region of interest on the first frame that just covers all of the identified feature regions in the sequence of feature regions, so as to determine a three-dimensional range of interest; and a control unit configured to control an aperture of the x-ray collimator based on the determined range of interest.