An aperture cooling structure (a cooling structure used for the open X-ray source) 10 comprises an aperture unit 31 formed with an aperture 33, a holder 34 for holding the aperture unit 31, and a heat dissipator 36 connected to the holder 34. The aperture 33 restricts an electron beam E from passing therethrough on an electron path 4 of an X-ray generator (open X-ray source). The heat dissipator 36 has a heat dissipation member 37 including a coolant flow path constituent part 41 and a heat dissipation member 38 including a coolant flow path constituent part 42. The coolant flow path constituent part 41 and the coolant flow path constituent part 42 are combined with each other, so as to construct a coolant flow path 43.