The present invention relates to a radiation generating apparatus which includes an envelope provided with a first window through which radiation is transmitted, a radiation tube housed in the envelope and provided with a second window through which the radiation is transmitted, the second window being located at a position opposite the first window, and an insulating fluid adapted to fill the space between the inner wall of the envelope and the radiation tube. Plural plates are arranged side by side between the first window including its periphery and the second window including its periphery, and overlapping one another with gaps between them. The gaps are formed among the plates, and thereby the withstanding voltage between the first window and second window is made larger.