A screen to be used when brazing a heat exchanger having a plurality of core plates formed from an aluminum alloy brazing sheet containing Mg and formed in a shape having a taper portion at the periphery which are heated and brazed under an inert gas atmosphere together with a base plate on the lower face side which is larger and thicker than the core plates, wherein: the screen is formed from metal as a tube enclosing a stacked body of the core plates, follows the outer border of the core plates such that the inner wall face of the tube has a specific minute gap between it and the tip edge of the taper portion, and is mounted onto the base plate, wherein the taper portion are stacked such that the taper portions touch together, characterised in that: the screen being configured separated into a plurality of pieces so as to enclose, from the perimeter, the stacked body of core plates assembled on the base plate, wherein the screen being configured separated in two into a first half portion and a second half portion so as to sandwich, from both sides, the stacked body of core plates assembled on the base plate, wherein at the boundary between the first half portion and the second half portion, the edge portion of one extends, being offset to the outside only by the plate thickness, and overlaps together with the other edge portion positioned on the inside, wherein the screen comprises swelled portions formed on the bottom edge portion of the screen, so that the screen can be located using locating pins provided on a base plate, which can be fitted into these swelled portions, in order to locate the stacked body of core plates on the base plate. the screen is formed from metal as a tube enclosing a stacked body of the core plates, follows the outer border of the core plates such that the inner wall face of the tube has a specific minute gap between it and the tip edge of the taper portion, and is mounted onto the base plate, wherein the taper portion are stacked such that the taper portions touch together, characterised in that: the screen being configured separated into a plurality of pieces so as to enclose, from the perimeter, the stacked body of core plates assembled on the base plate, wherein the screen being configured separated in two into a first half portion and a second half portion so as to sandwich, from both sides, the stacked body of core plates assembled on the base plate, wherein at the boundary between the first half portion and the second half portion, the edge portion of one extends, being offset to the outside only by the plate thickness, and overlaps together with the other edge portion positioned on the inside, wherein the screen comprises swelled portions formed on the bottom edge portion of the screen, so that the screen can be located using locating pins provided on a base plate, which can be fitted into these swelled portions, in order to locate the stacked body of core plates on the base plate. the screen being configured separated into a plurality of pieces so as to enclose, from the perimeter, the stacked body of core plates assembled on the base plate, wherein the screen being configured separated in two into a first half portion and a second half portion so as to sandwich, from both sides, the stacked body of core plates assembled on the base plate, wherein at the boundary between the first half portion and the second half portion, the edge portion of one extends, being offset to the outside only by the plate thickness, and overlaps together with the other edge portion positioned on the inside, wherein the screen comprises swelled portions formed on the bottom edge portion of the screen, so that the screen can be located using locating pins provided on a base plate, which can be fitted into these swelled portions, in order to locate the stacked body of core plates on the base plate.