A package mounting structure comprising a package mounted on a case, the package internally incorporating at least one of a high-frequency transistor, MIC and MMIC used in the microwave to millimeter-wave band, a base thereof being formed of metal and serving as ground, said package mounting structure further comprising: an electrically conductive sheet having thermal conductivity and exhibiting restorability, wherein: a package-bearing location and first and second circuit-board-bearing locations are provided on the case, the package-bearing location bearing the package and the first and second circuit-board-bearing locations being for bearing first and second circuit boards (52, 53); the package-bearing location has four sides, first and second opposite sides being the sides between the package-bearing location and the first and second circuit-board-bearing locations, respectively; right-angled step portions are present on the case at the first and second opposite sides between the package-bearing location and the first and second circuit-board-bearing locations; characterized in that slanted step portions are present on the case at third and fourth opposite said sides different from said first and second opposite sides; said sheet is laid on the case on the entirety of the package and circuit-board-bearing locations, the sheet extending over the slanted step portions; the package and the sheet are fastened together and attached to the case by two or more screws; said package has terminals (54c, 54d) for connection to microstrips of said circuit boards (52, 53); and said package mounting structure further comprises said first and second circuit boards (52, 53) installed on the sheet at the first and second circuit-board-bearing locations, and said terminals (54c, 54d) are connected to said microstrips of said circuit boards (52, 53). an electrically conductive sheet having thermal conductivity and exhibiting restorability, wherein: a package-bearing location and first and second circuit-board-bearing locations are provided on the case, the package-bearing location bearing the package and the first and second circuit-board-bearing locations being for bearing first and second circuit boards (52, 53); the package-bearing location has four sides, first and second opposite sides being the sides between the package-bearing location and the first and second circuit-board-bearing locations, respectively; right-angled step portions are present on the case at the first and second opposite sides between the package-bearing location and the first and second circuit-board-bearing locations; characterized in that slanted step portions are present on the case at third and fourth opposite said sides different from said first and second opposite sides; said sheet is laid on the case on the entirety of the package and circuit-board-bearing locations, the sheet extending over the slanted step portions; the package and the sheet are fastened together and attached to the case by two or more screws; said package has terminals (54c, 54d) for connection to microstrips of said circuit boards (52, 53); and said package mounting structure further comprises said first and second circuit boards (52, 53) installed on the sheet at the first and second circuit-board-bearing locations, and said terminals (54c, 54d) are connected to said microstrips of said circuit boards (52, 53). a package-bearing location and first and second circuit-board-bearing locations are provided on the case, the package-bearing location bearing the package and the first and second circuit-board-bearing locations being for bearing first and second circuit boards (52, 53); the package-bearing location has four sides, first and second opposite sides being the sides between the package-bearing location and the first and second circuit-board-bearing locations, respectively; right-angled step portions are present on the case at the first and second opposite sides between the package-bearing location and the first and second circuit-board-bearing locations; characterized in that slanted step portions are present on the case at third and fourth opposite said sides different from said first and second opposite sides; said sheet is laid on the case on the entirety of the package and circuit-board-bearing locations, the sheet extending over the slanted step portions; the package and the sheet are fastened together and attached to the case by two or more screws; said package has terminals (54c, 54d) for connection to microstrips of said circuit boards (52, 53); and said package mounting structure further comprises said first and second circuit boards (52, 53) installed on the sheet at the first and second circuit-board-bearing locations, and said terminals (54c, 54d) are connected to said microstrips of said circuit boards (52, 53).