A medical X-ray image detecting device wherein the fluorescent surface of a narrow belt-shaped X-ray fluorescen plate which emits light at the time of irradiation of X-ray through a slit is divided to rectangular units, optical fiber bundles are distributed alternately right and left and arranged vertically, the one end surface of each optical fiber bundle is attached to one of the divided fluorescent surface unit, the other end surface of each optical fiber bundle is attached to the image pickup surface of a solid-state image pickup device to make image formation possible, and the solid-state image pickup device is disposed behind an X-ray shielding member by which the slit is formed.