A transformer comprising: a first coil section formed by coiling a conductor pattern by one round or more 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 by one round or more, the coil being covered with an electric-insulating resin, the first ring being annular and being arranged on a first surface of the circuit board such that a through-hole of the first ring coincides with the insertion hole provided on the circuit board, a second ring composed of a coil formed by coiling a conductor plate by one round or more, the coil being covered with the electric-insulating resin, the second ring being annular and being arranged on a second surface of the circuit board such that a through-hole of the second ring coincides with the insertion hole provided on the circuit board, and a coupled part formed by covering a coupled position between the first ring and the second ring with the electric-insulating resin, wherein the respective coils (21, 22) of the second coil section are electrically coupled to each other at the coupled position; and core sections (30, 40) inserted through the insertion hole, the first coil section, and the second coil section, the core sections holding the circuit board from both sides of the circuit board so as to form a closed magnetic circuit that magnetically couples the first coil section and the second coil section, and the circuit board includes a slit, the coupled part being inserted and fitted into the slit so as to position the first coil section and the second coil section such that the insertion hole provided on the circuit board coincides with the through-holes of the first ring and the second ring of the second coil section. a first coil section formed by coiling a conductor pattern by one round or more 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 by one round or more, the coil being covered with an electric-insulating resin, the first ring being annular and being arranged on a first surface of the circuit board such that a through-hole of the first ring coincides with the insertion hole provided on the circuit board, a second ring composed of a coil formed by coiling a conductor plate by one round or more, the coil being covered with the electric-insulating resin, the second ring being annular and being arranged on a second surface of the circuit board such that a through-hole of the second ring coincides with the insertion hole provided on the circuit board, and a coupled part formed by covering a coupled position between the first ring and the second ring with the electric-insulating resin, wherein the respective coils (21, 22) of the second coil section are electrically coupled to each other at the coupled position; and a first ring composed of a coil formed by coiling a conductor plate by one round or more, the coil being covered with an electric-insulating resin, the first ring being annular and being arranged on a first surface of the circuit board such that a through-hole of the first ring coincides with the insertion hole provided on the circuit board, a second ring composed of a coil formed by coiling a conductor plate by one round or more, the coil being covered with the electric-insulating resin, the second ring being annular and being arranged on a second surface of the circuit board such that a through-hole of the second ring coincides with the insertion hole provided on the circuit board, and a coupled part formed by covering a coupled position between the first ring and the second ring with the electric-insulating resin, wherein the respective coils (21, 22) of the second coil section are electrically coupled to each other at the coupled position; and core sections (30, 40) inserted through the insertion hole, the first coil section, and the second coil section, the core sections holding the circuit board from both sides of the circuit board so as to form a closed magnetic circuit that magnetically couples the first coil section and the second coil section, and the circuit board includes a slit, the coupled part being inserted and fitted into the slit so as to position the first coil section and the second coil section such that the insertion hole provided on the circuit board coincides with the through-holes of the first ring and the second ring of the second coil section.