An imaging unit includes a housing having a wall portion in which a slit for passing radiation is formed, a scintillator having an input surface to which radiation passing through the slit is input, a first mirror that reflects scintillation light output from the input surface, and a line scan camera that detects scintillation light reflected by the first mirror. The scintillator is placed to make the input surface parallel to both the conveying direction and a line direction. The first mirror is positioned outside an irradiation region connecting the peripheral edge of the slit to the input surface of the scintillator.