PURPOSE: To make it possible to mount cassettes different in size to set automatically the field of X-ray irradiation, by providing holding pieces, which are moved vertically in accordance with the size of a cassette, and dogs which are slid in accordance with this size and detecting the length between moved dogs.

CONSTITUTION: Guide shafts 5 and 6 of a holding frame 3 are provided with holding pieces 7 and 8 vertically movably, and these pieces 7 and 8 are deflected to the outside by springs 22 and 23. A central operation substrate 13 is provided with an operation piece 14 and a detecting dog 14a slidably, and the dog 14a is connected to a dog 15 through a spring 16, and the dog 15 is connected to holding pieces 7 and 8 through wires 18a and 18b. When a cassette is not loaded, holding pieces 7 and 8 are deflected to the outside; but when a cassette is inserted, the operation piece 14 is moved to the right to draw holding pieces 7 and 8 toward the inside. Therefore, the cassette of a desired size is loaded. Since the length between dogs 14a and 15 is related to the size of the cassette, this length is detected by a detector 24 to discriminate the size, and a split mask and the field of X-ray irradiation are set automatically.