PURPOSE: To decrease an error due to a scattered ray or the like by arranging a radiant ray beam detector to a position on a straight line prolonged from an array detector and discriminating only a directly radiated beam so as to obtain position information.
CONSTITUTION: A radiant ray from a radiant ray source 1 becomes a sectorial radiant ray beam 3 via a collimator 2. An array shaped detector 5 comprising plural detection elements is arranged on a prolonged line of the beam, the difference of the absorption by a body 4 to be detected to obtain a radiant ray picture. A position detection pulse detector 6 is arranged at both sides of the detector 5 on the prolonged line of the detector 5 to detect a beam 3 and the position information with the detector 5 is sent to a control circuit. Based on the position information, the position of the collimator 2 or the detector 5 is controlled to optimize the relation between the beam 3 and the detector 5. The detector 6 consists of a pulse measuring device 7, an amplifier section 8 and a pulse discrimination section 9. Then the detector 6 applies pulse peak discrimination to separate the signal and a high energy signal from a point having lots of direct rays is used as position information to apply the control not susceptible to the effect of scattered rays.