PURPOSE: To correct the detection sensitivity of incident radiation at a visual field position by forming respective radiation detecting elements so that their detection surfaces are narrower in area toward a center part in the thickness direction of the incident radiation crossing the parallel providing direction of them.

CONSTITUTION: A fan beam X ray XB is transmitted through a sample PA and enters a detection cell of the radiation detector 10, but a fan beam X ray XB has a large intensity distribution DS decreased greatly nearby the edges. Thus, the arrival radiation decreases in intensity distribution on both ends surface sides in a slice direction S and the gathering electrode 12 of the cell is narrow in area toward the center part in the slice direction S and the sensitivity is suppressed lower at the center part than at the peripheral part, so a detection signal of nearly the same level with D' is extracted for C' as long as the same material is used at the end side P and center part O in the slicing direction S. Consequently, the unbalance of a radiation detection signal which is detected substantially can be corrected.