In a heat exchanger, an outside air passage, through which outside air flows, is provided between adjacent tubes of the refrigerant tube and the coolant tubes. Disposed in the outside air passage is an outer fin joined to at least one of the refrigerant tube and the coolant tube and configured to accelerate heat transfer between the fluids. A dimension of the refrigerant tube in a flow direction of the outside air is different from a dimension of the coolant tube in the flow direction of the outside air. Hence, with respect to the outer fin joined to both of the refrigerant tube and the coolant tube, an area of a joint surface between the refrigerant tube and the outer fin is different from an area of a joint surface between the coolant tube and the outer fin.