In an X-ray image intensifier, an incident window on which X-rays are incident is fixed to a support frame fixed to a glass vessel. The incident window has a dome portion and a flat portion around the dome portion, and is fixed to the support frame through an annular brazing sheet. The brazing sheet has brazing material layers. The brazing material layers are melted, thereby welding the brazing sheet, the incident window, and the support frame with each other. A groove is formed in the brazing sheet to form a brazing material puddle, so the brazing material will not reach the input screen of the incident window during melting. In an alternate embodiment, an anti-wetting layer is disposed on the incident window to prevent brazing material from wetting the input screen. In a further embodiment, the thickness of the X-ray incident window has a thickness that increases gradually from the center toward a peripheral portion thereof.