PURPOSE: To enable the effective removal of pile-up, by detecting the pile-up by comparing the sum total of detection outputs of a large amount of radioactive rays from a scintillator and the sum total of detection outputs through a non- linear circuit.

CONSTITUTION: The radioactive rays from each part of a scintillator 2 are detected by a photomultiplier 4 and a high level is suppressed in order to enhance space strain at the time of position counting by a non-linear circuit 5. The inputs and outputs of this circuit 5 are respectively added by adder circuits 7, 12 and the respective sum totals are compared by a comparator 13. In this case, pile-up is detected on the basis of the change in the compared result by the presence and absence of pile-up while the unblank of a display apparatus 10 is controlled through an unblank generating circuit 9 and the pile-up is effectively removed to prevent the deterioration of a resolving degree at the time of a high counting ratio.