An in-line waveguide divider divides power of an incoming high-frequency signal among openings. Amplification boards disposed on a base are provided for respective openings and are each connected in parallel with one another to the in-line waveguide divider. An in-line waveguide combiner includes openings formed correspondingly to the amplification boards, and is connected to the amplification boards. An electrically conductive amplifier cover includes walls formed to provide isolation between circuits of the amplification boards continuously from the in-line waveguide divider to the in-line waveguide combiner, and the entire surface of the amplification boards at the in-line waveguide combiner side is covered with the electrically conductive amplifier cover except openings and openings. Each of the amplification boards includes a waveguide-to-microstrip transition corresponding to the opening, an amplifier element, and a microstrip-to-waveguide transition corresponding to the opening.