To further improve heat exchange performance in a laminated core type heat sink in which a plate is formed by arranging a plurality of slits in parallel to a metal flat plate and laminating the plate in large numbers.Thickness T2 of a second plate 4b positioned on the second in the lamination direction from an end lid 9 to which a semiconductor is attached is made greater than thickness T1 of a first plate 4a other than the second plate 4b.