A heat exchanger core includes: a first passage row which is formed by a plurality of first passages; a plurality of first dividing walls separating the plurality of first passages from each other; a second passage row which is disposed adjacent to the first passage row and is formed by a plurality of second passages; a plurality of second dividing walls separating the plurality of second passages from each other; and a partition wall located between the first passage row and the second passage row, and separating the plurality of first passages and the plurality of second passages. (a) The partition wall has a greater section modulus in an orthogonal direction than either the first dividing wall or the second partition, or (b) a constituent material of the partition wall has a greater breaking strength than a constituent material of either the first dividing wall or the second dividing wall.