1. An image processing apparatus comprising: a storage configured to store first X-ray image data in which a blood vessel of an object is not visually enhanced, and second X-ray image data in which the blood vessel is visually enhanced; a specifier configured to specify a catheter image of a catheter included in a first fluoroscopic image by performing image processing of the first X-ray image data; an extractor configured to extract a blood vessel region included in a second fluoroscopic image by performing image processing of the second X-ray image data; a setting processor configured to set a region of interest based on the position of end point of the catheter; a display configured to automatically display a symbol corresponding to the region of interest on at least one of the first fluoroscopic image and the second fluoroscopic image; a thinning processor configured to calculate a positional relationship between an end point of the catheter image and a center line of the blood vessel region by thinning the catheter image and a blood vessel image; and a calculation processor configured to calculate blood flow information associated with the set region of interest; wherein the setting processor sets, as a plurality of the regions of interest, from the end point a plurality of positions separated by a predetermined distance along the center line, based on the positional relationship, using a result obtained by line tracing processing, the calculation processor calculates time density curve information on another region of interest on the blood vessel based upon one regions of interest that is on a closer side to the catheter among the plurality of the regions of interest, and if a branch of a blood vessel is detected in the line tracing processing, the setting processor traces the branch of the blood vessel, detects a region where a branch width of the branch is substantially large and sets the region as a region of interest candidate.