A transformer has a bar-shaped ferrite core, an inner winding wound directly around the ferrite core, an outer winding wound over the inner winding, and a dielectric shield surrounding the outer winding. The outer winding has its circumference covered by a dielectric sheath, and has an intermediate winding portion between its winding start end and its winding stop end. The dielectric sheath in the intermediate winding portion is spaced from each other along the axis of the ferrite core to leave thereat a gap which is filled with a molding material. By provision of the gap between the dielectric sheath of the outer winding, the molding material forming the dielectric shield can be easy to flow in between the dielectric sheath of the outer winding and the inner winding, avoiding the void from appearing between the inner and outer windings for attaining stable electrical characteristic.