Provided is an X-ray generator comprising an X-ray tube including a cylindrical body; an electron source in the body; a target at an end of the X-ray tube facing the electron source, the target generating X-rays by irradiation with electrons; a container in which the X-ray tube is arranged; insulating liquid filled between the X-ray tube and the container; and a holding member holding the body of the X-ray tube in the container, with a channel for the insulating liquid around the X-ray tube. The distance between the holding member and the end face at the end of the body in the direction in which the electron source and the target face is twice or more as large as the minimum width of the channel that is in contact with the outer surface of the X-ray tube at the end face side with respect to the holding member. This allows heat in the target to be quickly radiated, thus allowing X-rays to be generated stably for a long time.