An X-ray image diagnosis apparatus comprising: an imaging unit comprising an X-ray generation unit configured to emit an X-ray to which a subject is exposed, and an X-ray detection unit configured to detect the X-ray that has passed through the subject, in such a way that the X-ray generation unit and the X-ray detection unit are configured to face each other and it is possible to take an image of the subject placed on a top panel of a bed; a control unit adapted to collect mask images before a contrast medium is injected into the subject by performing control in such a way that a step movement process of at least one of the imaging unit and the top panel is carried out and the subject is irradiated with X-rays at a plurality of imaging stages, and to collect contrast images after a contrast medium is injected into the subject by performing control in such a way that the step movement process of the at least one of the imaging unit and the top panel is carried out and the subject is irradiated with X-rays at the plurality of imaging stages; and an image processing unit adapted to process image data taken at the plurality of the imaging stages, wherein the control unit is adapted to set, for the plurality of the imaging stages, a region of interest in a part of each imaging area of the subject which is at a location where a current-step imaging area and a next-step imaging area overlap, or at a location in the vicinity of the overlap when the mask images are taken, to measure a change of an image density in the contrast images in each of the respective regions of interest of the imaging area of the subject, to detect a flow of the contrast medium when the contrast images are collected, and to make at least one of the imaging unit and the top panel move to the next imaging stage on the basis of a result of the detection, and when an image of lower limbs of the subject is taken, the control unit is adapted to make the at least one of the imaging unit and the top panel move to the next imaging stage based on a change of the image density of a leg in which the contrast medium flows more slowly. an imaging unit comprising an X-ray generation unit configured to emit an X-ray to which a subject is exposed, and an X-ray detection unit configured to detect the X-ray that has passed through the subject, in such a way that the X-ray generation unit and the X-ray detection unit are configured to face each other and it is possible to take an image of the subject placed on a top panel of a bed; a control unit adapted to collect mask images before a contrast medium is injected into the subject by performing control in such a way that a step movement process of at least one of the imaging unit and the top panel is carried out and the subject is irradiated with X-rays at a plurality of imaging stages, and to collect contrast images after a contrast medium is injected into the subject by performing control in such a way that the step movement process of the at least one of the imaging unit and the top panel is carried out and the subject is irradiated with X-rays at the plurality of imaging stages; and an image processing unit adapted to process image data taken at the plurality of the imaging stages, wherein the control unit is adapted to set, for the plurality of the imaging stages, a region of interest in a part of each imaging area of the subject which is at a location where a current-step imaging area and a next-step imaging area overlap, or at a location in the vicinity of the overlap when the mask images are taken, to measure a change of an image density in the contrast images in each of the respective regions of interest of the imaging area of the subject, to detect a flow of the contrast medium when the contrast images are collected, and to make at least one of the imaging unit and the top panel move to the next imaging stage on the basis of a result of the detection, and when an image of lower limbs of the subject is taken, the control unit is adapted to make the at least one of the imaging unit and the top panel move to the next imaging stage based on a change of the image density of a leg in which the contrast medium flows more slowly.