PURPOSE:To make easy the ascertainment of a selected detecting field of X-rays and its locating operation by displaying the selected detecting field as an optical shadow on a surface of a photographing part of a body of a subject. CONSTITUTION:At the time of photographing, a desired switch among selective switches 4a-4c of a detecting field selecting unit 4 is closed, matching to a photographing part of a subject. Thereby, collecting electrodes 1a-1c are connected with an input portion of an automatic exposure control circuit through switches Sa-Sc, and, at the same time, projecting members 2a-2c of a mask 2 for projection become optically opaque by applying a voltage on the liquid crystals of the projecting members 2a-2c through liquid crystal drive control circuits 3a-3c. Then, when a lamp for setting up an illuminating field, which is arranged in a diaphragm for X-rays which is fitted on an irradiating opening of an X-ray tube, is lighted, the projecting members 2a-2c of the mask 2 are projected on an X-ray tube side surface of the photographing part of the subject.
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