A structure for encasing a package incorporating at least one of a high frequency transistor, MIC and MMIC for use in microwave through millimetric wave band and having a metallic base serving as the earth, wherein a recoverable conductive sheet of the same size as the base of the package and exhibiting a good thermal conductivity is placed on the case at a part for mounting the package and then the package is tightened together with the sheet using two or more screws thus encasing the package while pressing the sheet with a pressure of 10 N/cm<2> or above through tightening.
