To easily and quickly mount a heat sink to be freely detached as required by providing a fastener for detachably fix-connecting a heat sink and a substrate of a multi-chip module(MCM).

A heat sink 1 facing on an upper surface of a substrate of an MCM is so formed, in step, as to comprises a specified clearance corresponding to electronic parts of different heights on the substrate. Related to a clearance between an upper surface of each electronic part P and an inside surface of the heat sink 1, a heat-conductive member S is put in between for easy transfer of heat from each electronic part P to the heat sink 1. For maintenance of the MCM, etc., the heat sink 1 is detachably fixed to an MCM substrate B using a fastener. The fastener comprises four screws N screwed, for clamping, to the heat sink 1 with the MCM substrate B in between at, for example, four corners of a bottom surface of the MCM substrate B, by loosening and clamping operations with the screw N, attaching/detaching of the heat sink 1 is done very easily.