PURPOSE: To aim at the promotion of easiness for setting an optical detector to a device and the uniformization of photographic density, by transmitting the optical signal converted from X-rays at plural detecting fields with an optical fiber, and furthermore altering X-ray detecting efficiency at each detecting field at will.

CONSTITUTION: X-rays transmitting a subject 2 from an X-ray tube 1 is converted into an optical signal by each sensitizer paper of plural detecting fields 9, reflected at a reflecting layer, and incident into an optical fiber 12. The signal emitted out of the other end of the optical fiber is converted into an electric signal by an optical detector 7, and it is amplified by another amplifier 6 at each of these detecting fields. In consequence, X-ray detecting efficiency at every detecting field 9 is alterable at will. An exposure controller 5 standardizes the output electric signal at every detecting field 9, and at that point that an integrating quantity of the electric signal reaches to the equivalent valve of an X-ray exposure quantity corresponding to proper density, it outputs a exposure stopping signal. With this signal, an X-ray controller 4 controls the exposure for its stopping. With this constitution, facilitation of setting the optical detector to this device and uniformization of photographic density are promotable.