PURPOSE: To eliminate change of detection sensitivity and make possible provision of slits for scattering ray removal on the detection side by so constituting data collection mechanism that a radiation source and detectors always maintain the same position relation.

CONSTITUTION: The rotating circumference 2 of a radiation source T is centered at the subject center 0, and the rotating circumference 2 as the imaginary arraying circumference of detectors is divided by (n) (n is an integer) to determine points P1, P2. A group of detectors to be arrayed at the arcs P2, P2 of the imaginary arraying circumference 2 are arrayed on a circumference 3, forming a detection block Da. In this way, other n-1 pieces of detector blocks Db thru Df are formed, so thet the straight lines connecting their end parts and the radiation source T always run the equally divided points and move along with the rotating movement of the radiation source T while the relative position relations with the radiation source T are being maintained. The detector blocks Da thru Df are so formed that the distances from their adjoining ends up to the radiation source T vary, whereby the data loss at the respective boundaries of the detector blocks Da thru Df is reduced