A low-cost heat sink easy to assemble which is designed to be mounted on a power semiconductor module (5) through the medium of cooling water includes a base member (1), a heat sink body (2) superposed on the base member (1) to form in cooperation with the base member (1) a passage through which the coolant flows, and a bellows-like flow straightening plate (6) disposed between the power semiconductor module (5) and the base member (1) in physical contact with the power semiconductor module (5) on one hand and with the base member (1) on the other hand. The flow straightening plate (6) partitions the passage into a plurality of flow straightening channels (14A, 14B).