The cooling device comprises a heat exchange unit provided in a cooling water flow path. The heat exchange unit has a plate part in the form of a plate and multiple fins erected on the plate face of the plate part, and provided in the manner that the plate face of the plate part extends along the cooling water flow path and the cooling water runs through the space between the fins. The multiple fins are each in the form of a rhombic column and provided on the plate part in a lattice pattern in the manner that the longer diagonal of the rhombic column extends along the cooling water flow path.