control circuit 50 phase modulates the output level of the tube current level setting circuit 52 at a chopping period and produces a modulated signal this modulated signal is then supplied to the base of the inverting transistor 43 so as to control the chopping ratio the ac power of the rectangular waveform is controlled in accordance with the chopping ratio of the chopping ratio control circuit 50 and is supplied to the filament so as to obtain the preset tube current level as a result the filament can be stably heated thermionic emission from the filament corresponding to the temperature of the heated filament can be performed in fact when the tube voltage is applied to the two electrodes of the x ray tube the thermions are emitted and the preset tube current flows through the x ray tube therefore the x rays which have a dosage corresponding to the tube voltage as well as the tube current are irradated from the x ray tube when the filament of the x ray tube is heated under the preset conditions and a high voltage is applied across the x ray tube 25 the x ray is irradiated from the x ray tube 25 to the object 26.