A power stack includes cooling pipes and semiconductor modules which are alternately laminated. Each cooling pipe includes an inside space dissected into cooling passages in which coolant flows. Both surfaces of the semiconductor module in a laminating direction are brought into contact with surfaces of neighboring cooling pipes. The semiconductor modules are classified into a plurality of groups mutually differentiated in their heat generation rates. And, any two semiconductor modules belonging to the same group having the highest heat generation rate are spaced from each other so that a cooling pipe is not sandwiched between these semiconductor modules in the laminating direction.