1. An X-ray diagnosis apparatus comprising: an X-ray tube which generates X-rays; an X-ray collimator which variably narrows X-rays from the X-ray tube; an X-ray detector which detects X-rays transmitted through a subject; a rotating mechanism which rotates the X-ray tube around the subject together with the X-ray detector; an image reconstruction unit which reconstructs a three dimensional image on the basis of 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 which sets a region of interest on an image of at least one frame by the X-ray detector; and a control unit which controls an aperture of the x-ray collimator on the basis of the set region of interest.