A heat exchanger manufacturing method comprising: a welding step of disposing a first weld bead through a thickness of one of the inner and outer plate portions and fusing the first weld bead to other of the inner and outer plate portions for joining together the first and second case halves, to thereby provide the heat exchange tube; and a temporary tacking step of providing a temporarily-assembled end plate/tube unit by temporarily tacking the heat exchange tube to each of the end plates by means of a second weld bead and filling, with the second weld bead, a gap of a generally triangular shape defined, at each of the opposite end portions of the heat exchange tube, by an outer surface of the inner plate portion, an end surface of the outer plate portion and a corresponding one of the end plates.