PURPOSE: To improve heating adaptation at the time of focal point changing by connecting in parallel two filaments of a double-focal point X-ray tube with the secondary coil of a heating transformer through rectifiers having the mutually inverse polarity while impressing a positive and negative asymmetrical and monoperiodical excitation current with an inverted phase.

CONSTITUTION: A double focal point X-ray tube 1 has a filament 1a for a large focal point, a filament 1b for a small focal point and the positive pole 1c, while the secondary coil 5b of a heating transformer 5 is connected with the filaments 1a and 1b through the rectifiers 3 and 4, which are connected with the mutually inverse polarity. The DC voltage being rectified 9 from the power source 18 is impressed on a capacitor 8 for resonance serially connected with a primary coil 5a of the transformer 5. An excitation current exciting means 23 is controlled by a timing means 17 while inverting a phase of an excitation current to the let flow to the coil 5a. The current phase is detected by a phase detection means 16 basing on the detection signal of the current detection means 7 composing a feedback control means 22 to operate the means 17.