PURPOSE: To make it possible to perform adjustment for equalizing the outputs of photomultiplier tubes through preamplifiers, by arranging a radiation source to a position corresponding to one of plural photomultiplier tubes to select the output of the amplifier thereof.

CONSTITUTION: A radiation source 1 is arranged to the position corresponding to the photomultiplier having a number shown by the switch 11 in a large number of photomultipliers 4. In this case, the output of the preamplifier 5 of said photomultiplier tube 4 is selected by a multiplexer 6 to be applied to a crest analyser 9 through an integrator 8 and compared with the different set level due to a level generator 12 to perform crest discrimination. This discrimination result is counted by a cunter 10 and the displacement amount of a center peak is displayed to a figure display device 14 by a subtractor 13 to adjust the high voltage variable resistance of the photomultiplier tube 4 so as to bring the display value to zero. Therefore, adjustment can be performed to equalize the outputs of the photomultiplier tubes through the preamplifiers thereof and correction good in reproducibility can be simply performed with high accuracy.