A plurality of waveguide structures are loaded on a top surface opposed to a bottom surface of a metal case, on which a high-frequency circuit is mounted, a height, a width, and a length of each of the plurality of waveguide structures have dimensions corresponding to a quarter-wave of a cutoff frequency indicating a frequency band of a target electromagnetic wave to be blocked, and a width and a length of each of the plurality of waveguide structures have dimensions that allow only a high-frequency wave of a mode to propagate in the frequency band.