A chamfered portion configured to have a surface which is tilted to a side surface and a rear surface is formed between the side surface and the rear surface of a housing of an electronic cassette. The chamfered portion has a boundary portion having a predetermined range in the vicinity of a boundary including the boundary with the side surface; a boundary portion having a predetermined range in the vicinity of a boundary including the boundary with the rear surface, and the other portion which does not belong to these boundary portions. The chamfered portion needs to be smoothly inserted into a gap between a patient and an installation surface on which the patient lies on one's back. Therefore, an entire surface of these boundary portions is formed in a curved surface which protrudes outward from the housing.