Provided is an X-ray generator including an electron passage in an electron-passage forming member; and a target on an insulative substrate. The transmission X-ray generator irradiates the target with electrons that have passed through the electron passage to generate X-rays. The target is provided at a central region of the substrate; the electron passage accommodates a secondary-X-ray generating section that generates X-rays by irradiation with electrons reflected from the target; the secondary-X-ray generating section and the target are disposed so that both of X-rays generated by direct irradiation of the target with the electrons and X-rays generated by irradiation of the secondary-X-ray generating section with the electrons reflected from the target are radiated to the outside; and at least part of the peripheral region of the substrate has higher transmittance for the X-rays generated at the secondary-X-ray generating section than the central region of the substrate.