In a heat exchanger, a plurality of heat conductive pipes forming first passages are provided between an outer wall plate and an inner wall plate. Air flows through the first passages. Second passages are formed between the heat conductive pipes. An exhaust gas flows through the second passages. Each of the heat conductive pipes has opposite walls limiting the passage width of the first passage. The walls are formed along involute curves. The second passages are formed by providing adjacent heat conductive pipes along the involute curves.