A heat exchanger includes a plurality of heat exchange units. Each of the plurality of heat exchange units includes: an internal space in which a fluid to be heated flows, a plurality of gas vents penetrating the internal space in a non-communicating state and through which combustion exhaust gas flows, at least one inlet port, and at least one outlet port. At least the one inlet port and at least the one outlet port in each of the heat exchange units are disposed at both ends in a longitudinal direction of the heat exchange unit and are shifted (offset) in a lateral direction of the heat exchange unit.