An X ray diagnostic apparatus includes an X ray tube generating X rays, a first detector detecting the X rays, at least one second detector arranged in front of a first detection surface of the first detector and including a second detection surface narrower than the first detection surface and indicator points provided on a rear surface of the second detection surface, a projection data generation unit generating first projection data based on an output from the first detector, and a positional shift detection unit detecting a positional shift of the second detector relative to the first detector in accordance with an imaging direction by using the first projection data and a predetermined positional relationship between the points and detection elements in the second detector.