In a radiation generating apparatus of the invention, a radiation tube having a cathode for emitting an electron and an anode for generating a radiation by an irradiation of the electron emitted from the cathode is enclosed in a housing container filled with an insulating liquid. The apparatus has a mechanism in which a voltage for allowing the insulating liquid to flow can be applied between the cathode and the anode in a state where the cathode does not emit any electron. Even when the generation of the radiation is stopped, the insulating liquid can be efficiently sufficiently cooled without providing mechanical stirring means, and a high reliability is obtained even for intermittent use.