This invention relates to a package unit including a semiconductor package that houses a semiconductor chip, and a heat sink attached thereto. A peripheral wall section that surrounds a thermal junction member is provided on a stiffener. More preferably, a tip end section of the peripheral wall section is allowed to fit into a slit provided on the heat sink. The peripheral wall section may be provided on a heat spreader instead. It is possible to prevent the thermal junction member from falling outside without incurring addition of new components or new occupancy of the area on a system substrate.