An electromagnetic wave adjusting device includes a first substrate, a first conductive element, a first insulation layer, a second substrate, a second conductive element, a dielectric layer, and a conductive layer. The first conductive element is disposed on the first substrate. The first insulation layer is disposed on the first conductive element. The second conductive element is disposed on the second substrate. The dielectric layer is disposed between the first substrate and the second substrate. The first conductive layer is disposed on the first insulation layer and electrically connected to the first conductive element. The electromagnetic wave adjusting device includes an overlap area and a capacitance adjustable area. An overlap portion of the first conductive element and the second conductive element constitutes the overlap area, the capacitance adjustable area includes the overlap area, and at least part of the first conductive layer is disposed in the capacitance adjustable area.