PURPOSE: To achieve a smaller and less weight of a support device by reducing the number of members composing a pressureproof support device to simplify the structure in a pressureproof device for supporting a heat exchange element and a casing outer circumferential wall of a gas vertical flow type boiler such as waste heat recovery boiler.

CONSTITUTION: In a vertical flow type boiler in which a top beam hangs down inside a casing outer circumferential wall 40 and a group of heat exchange elements 1 is provided being supported vertically on a blocked support device, the support device is linked horizontally on a link plate and both ends thereof are fixed on the casing outer circumferential wall 40 to form a pressureproof support structure of a boiler. Thus, the support device and the link plate are made to function as horizontal tension member.