An antenna device includes an antenna element and an impedance conversion circuit connected to the antenna element. The impedance conversion circuit is inserted between the antenna element and a feeding circuit, and includes a first series circuit where a first coil conductor and a second coil conductor are connected in series, and a second series circuit where a third coil conductor and a fourth coil conductor are connected in series. The first and second coil conductors define a closed magnetic circuit through which a closed loop of a first magnetic flux passes, and the third and fourth coil conductors define a closed magnetic circuit through which a closed loop of a second magnetic flux passes. Consequently, the antenna device performs impedance matching with the feeding circuit in a wide frequency band.