The transformer 5 used for the present invention, comprises a core of E-shaped first and second core halves 5g, 5h coupled together in opposed relation to each other to form a closed magnetic circuit or circuits for supporting primary and secondary windings 5a, 5b wound around first and second core halves 5g, 5h; and a gap 5n defined between end surfaces 5l, 5m of first and second core halves 5g, 5h by spaced relation of end surfaces 5l, 5m to also form a reduced section area 5v by contacting the end surfaces 5l, 5m. When a large amount of electric current flows through resonant series circuit 14, reduced section area 5v comes to magnetic saturation to reduce an excitation inductance 5f of transformer 5, more increase excitation current ILP and thereby perform steady operation in any load condition. When a small amount of electric current flows through resonant series circuit 14, excitation inductance 5f of transformer 5 is increased to repress excitation current ILP, thereby reducing power loss during light or no load to improve power conversion efficiency during light load.