Scattered X-rays scattered by an object or a structure enter in a detector (a shift detector) for detecting the positional shift of an X-ray focal point and become a noise source, thereby deteriorating the positional shift detection precision. In particular, the estimation of the dose of scattered X-rays originating from the object is difficult prior to the measurement, and correction of the scattered X-rays is important in order to precisely calculate the positional shift of the X-ray focal point. In order to address this drawback, according to the present invention, a scattered X-ray detector 6 is provided which measures the dose of scattered rays entering in a shift detector 5 for detecting the positional shift of an X-ray focal point 9, and has a function that the output by the shift detector 5 is corrected using the scattered ray dose measured by the scattered X-ray detector.