A heat exchanger includes a refrigerant tube through which a refrigerant for absorbing heat from air flows, and a defrosting medium tube through which a coolant for supplying heat for defrosting flows. Two fins disposed on both sides of the refrigerant tube include a protrusion protruding toward an upstream side of the flow of air with respect to the refrigerant tube. The protrusion is provided with a clearance that allows melt water generated in the defrosting to flow on the upstream side of the refrigerant tube. The refrigerant tubes and the defrosting medium tubes can be alternately arranged in the upstream line. The protrusion protrudes such that the air can be introduced into an air passage from the side portion of the protrusion even when an end facing the upstream side of the flow of the air is closed with the core of frost.