1. An image processing apparatus comprising: a storage unit that stores data of a plurality of X-ray projection images generated through a angiographic imaging sequence; a generation unit which, based on the data of the plurality of X-ray projection images, generates a reference time density curve concerning a reference region which is set as a blood supply region for a target portion, and a plurality of time density curves concerning a plurality of perfusion regions which are set as a blood supplied region in the target portion; a measurement unit which measures a plurality of delay times for which a contrast medium reaches the plurality of perfusion regions from the reference region, from relationships between the reference time density curve and the plurality of time density curves; a calculation unit which calculates, respectively for the plurality of perfusion regions, a plurality of first indices expressing correlations of the plurality of time density curves to the reference time density curve within a period determined on the basis of the delay times; and a first image generation unit which generates a first image expressing spatial distribution of the plurality of first indices.