1. An image processing apparatus comprising: a display unit configured to display an X-ray image; a data storage unit configured to store a correspondence table describing a relationship between blood vessel contrasts and reference contrast medium concentrations, reference injection rates, reference blood vessel diameters, and reference X-ray conditions; an input unit configured to receive input information including a planned value of a concentration of a contrast medium planned to be injected, a planned value of injection rate, a planned value of a blood vessel diameter, and an X-ray condition planned to be applied when an angiographic image is captured; an acquisition unit configured to acquire a blood vessel contrast by referring to the correspondence table based on the planned value of the concentration of the contrast medium planned to be injected, the planned value of injection rate, the planned value of blood vessel diameter, and the X-ray condition; a calculation unit configured to calculate a predictive pixel value of a region of interest set on the X-ray image based on the acquired blood vessel contrast and a pixel value of the region of interest; and a control unit configured to control the display unit to display the X-ray image while superimposing a partial image having the calculated predictive pixel value on the region of interest.