1. An X-ray diagnostic apparatus comprising: an X-ray tube which generates an X-ray; an X-ray detector which detects the X-ray which penetrates a subject; a storage memory which stores data of a plurality of X-ray projection images generated through a angiographic imaging sequence by the X-ray detector; a processor configured to generate, based on the data of the plurality of X-ray projection images, 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; measure a plurality of delay times for the plurality of perfusion regions respectively based on relationships between the reference time density curve and the plurality of time density curves; calculate respectively for the 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 after elapse of the delay times and a plurality of second indices expressing correlations of the plurality of time density curves to the reference time density curve within a period after elapse of an equal delay time in all of the perfusion regions; generate a first image expressing spatial distribution of the plurality of first indices in the perfusion regions and a third image expressing spatial distribution of the plurality of second indices in the perfusion regions; and a display which displays the first image together with the third image.