According to one embodiment, an antenna device includes a first element, a stub, and an open end element. The first element has a folded monopole structure in which a conductor is folded at a folding portion to form a forward portion and a backward portion. A base end of the forward portion is connected to a feeding point, and a distal end of the backward portion is connected to a ground via a first lumped parameter. The stub is provided between the forward portion and the backward portion of the first element so as to shunt the forward portion and the backward portion. The open end element includes a conductor placed in parallel to the first lumped parameter. A base end of the conductor is connected between the stub of the backward portion of the first element and the ground, and the distal end of the conductor is open.