Abstract: Being a heat pipe heat exchanger wherein a group of heat pipes are arranged in box form and the central part thereof is partitioned, a high temperature fluid being let to flow into one and a low temperature fluid into the other one, respectively, so that, by the specific properties of the heat pipes, the heat given from the high temperature fluid is transferred to the low temperature fluid through the sealed-in fluid in the heat pipes, a plurality of heat pipes of which those on at least the high temperature fluid passage side are bare pipes are arranged to extend over both passages, and the heat pipes on the high temperature fluid passage side are inserted in finless outer pipes. At least the outer surface of these finless outer pipes is treated for resistance to corrosion. Also, these finless outer pipes and the heat pipes are joined by a heat conductive material, so that heat pipes can be easily demounted, and, therefore, the efficiency of the heat exchanger can be varied as required.