A heat exchanger element includes at least two honeycomb structures arranged serially and each including a cell structural portion having cells separated and formed by partition walls containing SiC and functioning as passages which extend from one end face to the other end face and which a first fluid passes through, and an outer peripheral wall disposed on the outer periphery of the cell structural portion. The first fluid passes through the cells of the honeycomb structures without leaking out of the cells or mixing. The cell structural portions of at least a pair of the honeycomb structures adjacent to each other among the honeycomb structures arranged serially are disposed with a gap, and the first fluid passing through the cells mixes together between end faces forming the gap.