A BGA substrate which has a back surface to which a heat radiating plate is attached and an opening for accommodating a relay wiring substrate therein, which is provided in the center of its surface, is used. The relay wiring substrate to which an ASIC chip and a memory chip are flip-chip connected, is bonded to the heat radiating plate in the opening with a thermal conductive bonding material. Further, each of the back surfaces of the ASIC chip and the memory chip is connected to a metal cap for sealing the opening through a thermal conductive material interposed therebetween.