PURPOSE: To eliminate the effect of afterglow by providing an afterglow signal generating circuit which generates the signal corresponding to the afterglow of a scintillator.

CONSTITUTION: The output signal of a photoelectric transducer 10 based on afterglow becomes signal waveform 1. This signal 1 is superposed to the correct signal based on the light emission of a scintillator 9 owing to incidence of X-ray beams and is supplied to an amplifier 12. The signal amplified by the amplifier 12 is supplied to an afterglow signal generating circuit 13. The afterglow signal is added to the output signal of the amplifier 12 by an adder 14 after it has been inverted by a signal inversion circuit 15 and this turns out that the signal based on the afterglow contained in an X-ray detector 11 is reduced, thus only the correct signal free from the effect of the afterglow is supplied to a computer 16.