Abstract: Fins are each formed with a flat shape in a height direction, which is orthogonal to a flow direction of coolant in a coolant passage, and a plurality of the fins are provided intermittently along virtual waveforms extending in the flow direction and making a plurality of rows in a width direction that is orthogonal to the height direction and to the flow direction. An upstream portion of a first fin provided in a first row overlaps with the position in the flow direction f of a downstream portion of a second fin provided in a second row adjacent to the first row. Furthermore, the downstream portion of the first fin overlaps with a position in the flow direction of an upstream portion of a third fin provided in the second row.