Medical-image-processing device, x-ray diagnostic device, medical-image-processing method, and x-ray diagnostic method

A medical-image-processing device according to an embodiment is equipped with an image storage unit, an image acquisition unit, an information addition unit, and an image synthesis unit. The image storage unit stores first x-ray image data for a plurality of frames corresponding to a region including a region of interest, and corresponding to a plurality of different time phases. An image acquisition unit sequentially acquires second x-ray image data for a plurality of frames corresponding to the region of interest, and corresponding to the plurality of time phases. The information addition unit adds information expressing the plurality of time phases to the first and second x-ray image data. The image synthesis unit sequentially synthesizes the first and second x-ray image data for which the time phases correspond, on the basis of the information expressing the plurality of time phases which was added to the first and second x-ray image data.