In a heat exchanger, a second fluid flowing space communicating with second tubes is formed to be divided from a first tank space within a tank unit forming a first tank space that collects or distributes a refrigerant. The tank unit is defrosted by a coolant flowing in the second fluid flowing space, which is higher in temperature than the refrigerant flowing in the first tank space. With this configuration, heat from the second fluid flowing in the second fluid flowing space included in the tank unit is effectively transferred to a portion of the tank unit which is likely to be frosted.