A transformer capable of readily ensuring a predetermined insulation distance with no increase in the overall size of the transformer and further capable of reducing the number of constituent parts to achieve cost reduction. According to the present invention, the transformer includes a first bobbin having a wire winding section and a flange formed at an end of the wire winding section, a first coil wound around the wire winding section of the first bobbin, a second coil disposed coaxially with the first coil and adjacent to the flange, and a core disposed around the outer circumferences of the first and second coils to form a closed magnetic circuit, and a tubular protrusion that axially extends and surrounds the outer circumference of the second coil is formed along an outer circumferential portion of the flange of the first bobbin.