The present invention is directed to a heat exchanger, such as an evaporator for an automotive air conditioning refrigeration circuit. The heat exchanger includes a serpentine tube in which the interior space is divided by a plurality of parallel partition wall into a corresponding plurality of essentially parallel passages through which refrigerant fluid flows, and inlet and outlet mechanisms connected to the two opposite terminal ends of the serpentine tube, respectively. The inlet mechanism comprises a cylindrical header pipe portion which is fixedly disposed at one terminal end of the serpentine tube along the depth of the serpentine tube, and a separate cylindrical conducting pipe portion of which one terminal end is connected at a right angle to the inner side section of the air outflow region of the header pipe portion. Preferably, the terminal end of the conducting pipe portion includes a flat tube region at its terminal end, and further includes an axial movement preventing mechanism for preventing axial movement of the conducting pipe portion after insertion of the flat tube region of the conducting pipe portion into the header pipe portion.