An interface module-mounted LSI package has an interposer equipped with a signal processing LSI, an interface module for signal transmission, mechanically connected to the interposer and electrically connected to the signal processing LSI, and a heat sink which is in contact with both the signal processing LSI and the interface module, and radiates heat of the signal processing LSI and the interface module. The LSI package has a gap, which serves as a heat resistor portion between the heat radiating portion for the signal processing LSI and the heat radiating portion for the interface module.