PURPOSE: To irradiate only X-ray on prescribed irradiating width even if the energy of electron beam is high by providing an electronic beam absorbing member composed of lightweight conduction materials on the circumferential part of a conical filter part of a flattening filter.

CONSTITUTION: A flattening filter 5 is composed of absorbing member 23 of an electron beam A which makes the shape of the filter 5 approximately circu lar plate as a whole so as to bury a conical filter part 22 which is composed of heavy metal such as lead and copper alloy and the circumferential part thereof in order to flatten the distribution of X-ray B on a base 21 which is made a support part. The member 23 is composed of sufficiently lightweight conduction materials such as carbon as compared with the metal of the filter part 22 both metals are electrically connected each other and jointed to the earth part of X-ray generating device. When electron beam energy is high, the thickness of the absorbing member 23 is thickened because the directivity of X-ray is heightened and total thickness of a target 1, a holder 2, cooling water 3 and the absorbing member 23 is thicker than the range. Therefore, planned X-ray irradiation can correctly be performed.