A transformer includes: a first coil section formed by coiling a conductor pattern in a planar state around an insertion hole provided on a circuit board; a second coil section that includes a first ring composed of a coil formed by coiling a conductor plate, the coil being covered with an electric-insulating resin, a second ring composed of a coil formed by coiling a conductor plate, the coil covered with the electric-insulating resin, and a coupled part formed by covering a coupled position between the first ring and the second ring with the electric-insulating resin; and core sections forming a closed magnetic circuit that magnetically couples the first coil section and the second coil section.