Oil cooler is provided to include: a number of core plates each of which has three oil pass holes where oil flows and three cooling water pass holes where cooling water flows; heat-exchanging section where core plates are laminated to define inter-plate oil flow passage and inter-plate cooling water flow passage alternately between an adjacent pair of core plates, in which oil and cooling water can mutually independently flow in direction perpendicular to core plate lamination direction while changing its flow direction by U-turn thereby proceeding in core plate lamination direction as a whole; one end part located at one side of core plate lamination direction and provided with both oil inlet and oil outlet; and the other end part located at the other side of core plate lamination direction and provided with both cooling water inlet and cooling water outlet.