A heat exchanger includes plural heat exchange units arranged in series in a flowing direction of external fluid. A tube of the heat exchange units has a tube body and a protrusion. A dimension of the protrusion in a tube stacking direction is smaller than a dimension of the tube body in the tube stacking direction. A dimension of the protrusion in an air flowing direction is larger than a thickness of the tube body. An outer fin is joined to both the upstream tube and the downstream tube arranged in the air flowing direction. The protrusion of the upstream tube is connected to an upstream end of the tube body in the air flowing direction. The protrusion of the downstream tube is connected to a downstream end of the tube body in the air flowing direction.