COIL UNIT, POWER TRANSMISSION DEVICE, EXTERNAL POWER SUPPLY DEVICE, AND VEHICLE CHARGING SYSTEM

A coil unit includes a resonant coil (110) establishing electromagnetic field resonant coupling with an externally provided external resonant coil. The resonant coil (110) includes an outer side coil (115) extending along a circumference of a first winding center (O1), centered about the first winding center (O1), and an extension extending from the outer side coil (115) into a region surrounded by the outer side coil (115). The resonant coil (110) is formed such that an anti-node (AM1) of the resonant coil (110) that is an area of an anti-node of AC current flowing through the resonant coil (110), is located at the extension when electromagnetic field resonant coupling is established between the external resonant coil (240) and the resonant coil (110).