To provide a riser structure for a circulating fluidized bed incinerator, which enables efficient incineration of objects to be incinerated and size reduction in the furnace.

In the circulating fluidized bed incinerator, primary air is supplied for fluidization, to a fluidizing medium S which is filled in a riser 1, followed by supplying secondary air from a given position to permit a portion of the fluidizing medium S to ascend through the riser 1 at the superficial velocity more than the terminal velocity. At the same time, objects W to be incinerated are supplied to the medium that has been fluidized, so that the objects are subjected to burning and decomposition with the medium that has been fluidized and the fluidized medium which has been permitted to have a velocity higher than the terminal velocity. Thereafter the resultant is separated by a hot cyclone 7 into a fluidizing medium and an exhaust gas containing burned ash, and the separated fluidizing medium is collected to a loop- seal 9 through a downcomer 8; the collected fluidizing medium is then supplied with flowing air and returned to the riser 1; and there is provided a region 1A, having an enlarged cross-sectional area whose diameter is larger than the inner diameter of a part of the riser 1, which is positioned lower than the secondary air supply position.