Provided is an X-ray generating apparatus, which includes: an X-ray generating tube configured to emit X-rays through a first window; an outer case configured to contain the X-ray generating tube and provided with a second window transmitting the X-rays at a position facing the first window; an insulating fluid with which an unoccupied space of the outer case is filled; an insulating member located between the first window and the second window and provided with an opening in an irradiation area of the X-ray through the first window; and an insulating third window removably fit into the opening of the insulating member, wherein a linear expansion coefficient of the third window is greater than a linear expansion coefficient of the insulating member, and the third window and the first window face each other via a gap through which the insulating fluid is flowable.