A radiation generating unit includes a radiation tube that has a vacuum chamber that has a cathode and anode at both ends of an insulating tubular member and is arranged inside a storage container filled with an insulating liquid in a state in which the radiation tube is arranged inside an insulating outer casing tube with a gap from a surrounding, wherein walls which partition the gap between the radiation tube and outer casing tube are provided while allowing a flow of the insulating liquid between the cathode side and anode side of the radiation tube and leaving in the gap a flow path which does not linearly continue to the cathode side and anode side.