A portable detector panel includes an X-ray detector assembly having an X-ray detecting surface on its surface, a box-like case that houses the X-ray detector assembly therein and whose upper part that is opposite to the X-ray detecting surface is X-ray transmissive, and a buffer member that is arranged between the inner side wall of the case and the X-ray detector assembly, is made of a hard material, and has a flexible shape with respect to the movement of the X-ray detector assembly in the direction generally parallel to the X-ray detecting surface.