PURPOSE: To invariably allow the quick rising without applying an overload to an inverter circuit element by switching to the control by an optimum control means at the time of rising of an inverter and to the control by a feedback means subsequently.

CONSTITUTION: The optimum value outputted from an optimum control means 12 is fed to a switching transistor TR1 via a switching control means 13 by the switching action of the switching control means 13 at the time of rising, and the TR1 performs a switching action. DC high-voltage outputs applied to an X-ray tube 4 from output side rectifying circuits 7A and 7B have the fast rising time, the overshoot is prevented, and the TR1 and transistors TR2 and TR3 for inverter are not overloaded. When the DC high-voltage output becomes the fixed value, the switching control means 13 fed with the feedback signal outputted from a tube voltage detecting circuit 10 in response to this DC high-voltage output switches the control of the TR1 to the control by a feedback control means 15.