1. An X-ray photographing system for emitting X rays from X-ray emitting means toward a patient injected with a contrast medium, moving the patient and X-ray emitting means relative to each other along the flow of the contrast medium, and providing images of a wide area of blood vessels of the patient, comprising: tracing means for tracing trace points each corresponding to an end of an objective blood vessel according to image concentration data of images picked up at a predetermined relative position of the patient and X-ray emitting means; movement vector setting means for setting a movement vector for each of the trace points; movement decision means for selecting the largest one of the movement vectors and determining whether or not the selected vector extends beyond a zone defined around the start of the corresponding trace point; movement calculation means for calculating relative movements of the patient and X-ray emitting means according to the selected vector when said movement decision means determines that the selected vector extends beyond the zone; and driving control means for relatively driving the patient and X-ray emitting means according to the calculated relative movements.