An antenna device includes an antenna body, a circuit board, a joint, a transmission line conductor, and a line conductor. The antenna body includes a magnetic body and a conducting wire wound around the magnetic body in a spiral shape. The joint is disposed on the circuit board and coupled to an end of the conducting wire. The transmission line conductor is coupled to the joint. The line conductor is coupled to one of the end of the conducting wire and the transmission line conductor. At least one of a pattern and a length of the line conductor is changeable to adjust an equivalent impedance value of the antenna body.