Abstract: The device relates to an oil cooler and the object thereof is to provide an oil cooler with an improved mounting strength of water pipes against external stresses. A core portion 33 is formed by alternately stacking a plurality of plates 35, 37 and arranging cooling water passages 39 and oil passages 41 alternately between these plates 35, 37. On an upper portion of the core portion 33 is an oil tank 57 that is formed of an upper plate 47 and a cover plate 55. The upper plate 47 has a cooling water passage holes 43 communicating with the cooling water passages 39 and the oil passages 41. The cover plate 55 has water pipe insertion holes 51, 53 so as to correspond to the cooling water passage holes 43. The water pies 59, 61 are inserted into the cooling water passage holes 43 of the upper plate 47 by causing the water pipes 59, 61 to pass through the water pipe insertion holes 51, 53.