A scintillator attachment structure includes an opening formed in a first side wall portion of a housing and a scintillator holder that holds the scintillator and includes a holder portion fitted in the opening. The scintillator holder can be attached to and detached from the housing. While the scintillator holder is attached to the housing, a predetermined angle is formed between the scintillator held by the holder portion protruding from the first side wall portion into the housing and the front surface mirror in the housing.