A support structure supports a fluidized bed firing boiler disposed within a cylindrical pressure vessel and operated while the inside of a fluidized bed furnace is kept pressurized. A main body of the pressure fluidized bed firing boiler disposed within the pressure vessel is divided into a suspended section suspended from a support beam disposed at an upper interior portion within the pressure vessel, and a bottom-supported section supported by a support beam disposed at a lower interior portion of the pressure vessel. Also, a metallic expansion joint in the form of an undulated plate is provided at a point of engagement between the suspended section and the bottom-supported section.