Variation in inductance and leakage inductance between the secondary windings is reduced thereby making lamp currents uniform and thus preventing the occurrence of uneven brightness. A two-output inverter transformer comprises a bobbin having a primary winding wound around the middle and secondary windings wound on both sides thereof and having a pole-like core inserted into its winding shaft. A rectangle frame-like core is placed to surround the primary and secondary windings such that the pole-like core is opposite the rectangle frame-like core with a gap sheet interposed at either end. Their sizes are set such that L2-L1>2 t, where L1 is the length of the rectangle frame-like core, L2 is the length of the pole-like core, and t is the thickness of the gap sheet, and they are so combined that the pole-like core protrudes evenly from the opposite ends of the rectangle frame-like core.