An antenna device includes an impedance converter circuit connected to a feeder circuit, an impedance-conversion-ratio adjustment circuit, and an antenna. The impedance converter circuit includes a first inductance element and a second inductance element, which are coupled to each other through magnetic fields, so as to provide an autotransformer circuit. The impedance-conversion-ratio adjustment circuit includes a third inductance element which is series-connected between the impedance converter circuit and an antenna port, and a capacitance element which is shunt-connected between the antenna port and ground. The impedance-conversion-ratio adjustment circuit corrects an impedance conversion ratio of the impedance converter circuit in accordance with a frequency band.