A stack type heat exchanger includes a plurality of first plates and a plurality of second plates. At least one of the respective first plates and the respective second plates has a protrusion protruding from a main body of the first plate or the second plate toward a first flow path, the protrusion being located at a peripheral portion of a tank space in the first flow path. The first plate and the second plate are joined to each other through the protrusion. The protrusion has a top portion and a side wall portion. A part of the side wall portion adjacent to the tank space has a thick structure portion, an entire thickness of the thick structure portion being thick in a direction perpendicular to the stacking direction.