A heat exchanger includes: heat transfer tubes aligned with one another; a header connected to end portions of the heat transfer tubes; and fins joined to the heat transfer tubes. When viewed in a longitudinal direction of the header and when the heat exchanger is used as an evaporator, the header is divided into: a circulation space including a first space in which refrigerant flows in a first direction along the longitudinal direction of the header and a second space in which the refrigerant flows in a second direction opposite to the first direction along the longitudinal direction; and an insertion space into which the heat transfer tubes are inserted. The header includes: a circulation division plate that divides the first space from the second space; and an insertion space forming plate that divides the circulation space from the insertion space.