To provide an apparatus whose X-ray detector has a specified positional relationship and is not counter-supported by an X-ray tube assembly, which detects the position of the X-ray detector set behind a subject and the front side of the subject, calculates the subject's body thickness from their measurements, and enables to conditions to take an X ray be configured or calibrated.

A receiver 11 of a magnet sensor and an ultrasound range finder 21 are arranged adjacent to an X-ray tube assembly 5 of an X-ray apparatus; and transmitters 12 and 13 of the magnet sensor are mounted, with an X-ray image receiving region between them, at prespecified positions of an X-ray detector 8 which is portable and can be set at an optional position. After the X-ray detector 8 is set behind a subject, the magnet sensor is made to work by a CPU 30 to detect the front side position of the X-ray detector touching the subject, and the front side position of the subject is detected by the ultrasound range finder 21, so that the subject's body thickness is calculated from these detected values. The subject's body thickness calculated is used to calibrate default X-ray photographing conditions of the standard body.