A heat exchanger comprising a plurality of heat transfer units, each of the heat transfer units having a plurality of heat transfer channel portions and a plurality of auxiliary heat transfer portions, each of the heat transfer channel portions and each of the auxiliary heat transfer portions extending in a first direction, the heat transfer channel portions and the auxiliary heat transfer portions being arranged in a second direction, the second direction intersecting with the first direction or being perpendicular to the first direction, wherein; in each of the heat transfer units, when seen in the first direction, a first auxiliary heat transfer portion that is one of the auxiliary heat transfer portions is formed at an end portion in the second direction, the heat transfer units are arranged in a third direction, the third direction being different from both of the first direction and the second direction, in each of the heat transfer units, a first length of the first auxiliary heat transfer portion to a heat transfer channel portion that is adjacent in the second direction is larger than a distance between heat transfer channel portions that are adjacent to each other in the second direction; characterized in that positions of the heat transfer channel portions of one of the heat transfer units in the second direction and positions of the auxiliary heat transfer portions of an adjacent one of the heat transfer units in the second direction are arranged so as to overlap, such that, when seen in the first direction, the heat transfer channel portions and the auxiliary heat transfer portions of the heat transfer units are arranged in a staggered pattern; and, the heat transfer channel portions and the auxiliary heat transfer portions of each of the heat transfer units are integrally formed by extrusion of aluminum. each of the heat transfer units having a plurality of heat transfer channel portions and a plurality of auxiliary heat transfer portions, each of the heat transfer channel portions and each of the auxiliary heat transfer portions extending in a first direction, the heat transfer channel portions and the auxiliary heat transfer portions being arranged in a second direction, the second direction intersecting with the first direction or being perpendicular to the first direction, in each of the heat transfer units, when seen in the first direction, a first auxiliary heat transfer portion that is one of the auxiliary heat transfer portions is formed at an end portion in the second direction, the heat transfer units are arranged in a third direction, the third direction being different from both of the first direction and the second direction, in each of the heat transfer units, a first length of the first auxiliary heat transfer portion to a heat transfer channel portion that is adjacent in the second direction is larger than a distance between heat transfer channel portions that are adjacent to each other in the second direction; characterized in that positions of the heat transfer channel portions of one of the heat transfer units in the second direction and positions of the auxiliary heat transfer portions of an adjacent one of the heat transfer units in the second direction are arranged so as to overlap, such that, when seen in the first direction, the heat transfer channel portions and the auxiliary heat transfer portions of the heat transfer units are arranged in a staggered pattern; and, the heat transfer channel portions and the auxiliary heat transfer portions of each of the heat transfer units are integrally formed by extrusion of aluminum.