According to one embodiment, a planar antenna device includes a support portion, a first plate-shaped movable portion, a second plate-shaped movable portion, a third plate-shaped movable portion, and a pair of plate-shaped antenna units. The support portion is formed to have a plate shape. The first plate-shaped movable portion is supported by the support portion and is rotatable around a first axial line. The second plate-shaped movable portion is supported by the first plate-shaped movable portion and is rotatable around a second axial line. The third plate-shaped movable portion is supported by the second plate-shaped movable portion and is rotatable around a third axial line. The pair of plate-shaped antenna units is turnably supported by the third plate-shaped movable portion and is switchable between a closed state and an open state.