A radiation imaging apparatus includes an internal unit having a radiation detector arranged to convert a radiation that is passed through a subject into electric signals. A base plate is arranged to support the radiation detector. A case having a rectangular parallelepiped shape is arranged to accommodate the internal unit. A fitting member is interposed between an inner wall of the case and an end portion of the internal unit, and fitted to the inner wall of the case and the end portion of the internal unit in a planar view as seen from an incident direction of the radiation.