PURPOSE: To furnish a furnace wall support structure in a fluidized bed boiler which can follow the relative displacement of downcomers and a backstays and transfer smoothly to the backstays a pressure acting on a furnace wall which connects the downcomers.

CONSTITUTION: A boiler main body 1 stored in a pressure vessel 4 is constructed of a plurality of downcomers 10 extending vertically at corner parts of a boiler, a flat-surface-shaped furnace wall 12 which has vertical water tubes 11a and fins 11b connecting the water tubes and connects the downcomers 10 being adjacent to each other and backstays 14 surrounding the furnace wall with a space between them. The downcomer 10 and the backstay 14 are connected through the intermediary of a first horizontal member 22 having vertical pins 21 at the opposite end parts and being parallel virtually to the furnace wall and, moreover, the intermediate positions of the furnace wall and the first horizontal member being near the corner part are connected through the intermediary of a second horizontal member 25 having vertical pins 24 at the opposite end parts.