1. An image processing system comprising: an input unit that receives setting of a landmark in first medical image data obtained by capturing a certain tissue of a subject; an output unit that outputs data including information of a position of the landmark in the first medical image data as output data; an acquiring unit that receives the output data and acquires, based on one or a plurality of pieces of second medical image data obtained by capturing the certain tissue of the subject in one or a plurality of capturing directions, the one or the plurality of pieces of second medical image data corresponding to the respective capturing directions and the position of the landmark read from the output data, three-dimensional position information of the landmark in a three-dimensional capturing space of the second medical image data; and a display unit that displays image data obtained by superimposing the first medical image data on the second medical image data of the certain tissue, based on the three-dimensional position information of the landmark.