A sealing structure for multi-chip modules stable in cooling performance and excelling in sealing reliability is to be provided. The under face of a frame 5 compatible with a wiring board 1 in thermal expansion rate is fixed with solder 8 to the face of the wiring board 1 for mounting semiconductor devices 2; a rubber O-ring 15 is placed between the upper face of the frame 5 and the under face of the circumference of an air-cooled: heat sink 7; the plastic member 6 making possible relative sliding is placed between the upper face of the circumference of the heat sink 7 and the upper frame 10; the upper face of a plastic member 6 is restrained with the inside middle stage of an upper frame 10; and the lower part of the upper frame 10 and the frame 5 are fastened together with bolts 9.