1. An image processing apparatus comprising: a display unit configured to display an X-ray image; a data storage unit configured to store a first correspondence table describing a relationship between blood vessel contrasts and reference contrast medium concentrations 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 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 reference contrast medium concentrations and the reference X-ray conditions in the first correspondence table based on the planned value 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.