An X-ray detecting device for detecting characteristic X-rays emitted from a substance to be detected, which device comprises a vessel having a window defined therein for the passage of the characteristic X-rays emitted from the substance, therethrough into the vessel, and an X-ray detector accommodated within the vessel for the detection of the characteristic X-rays entering the vessel through the window. A portion of the vessel positioned on one side closer to the window and away from a sensing surface of the X-ray detector, upon which the characteristic X-rays impinge, and a peripheral lip area of the vessel defining the window have at least a respective area which is exposed to the interior of the vessel. This respective area is formed with a material which does not contain said substance.