A ground plane is disposed in a dielectric substrate or on the top surface of the dielectric substrate. A high-frequency semiconductor device is mounted on the bottom surface of the dielectric substrate. A shield structure that is provided in a space closer to the bottom surface than the ground plane is surrounds the high-frequency semiconductor device from below and sideways of the high-frequency semiconductor device and is connected to the ground plane. An opening is formed in the shield structure. A radiation-structure portion causes a high-frequency signal output by the high-frequency semiconductor device to be radiated through the opening.