To provide a riser structure for a circulating fluidized bed incinerator, which enables energy saving by reducing capacity of a blower for supplying primary air, reduction of noise and oscillation, and reduction in the height of a riser.

In the circulating fluidized bed incinerator T, primary air is supplied for fluidization, to a fluidizing medium S which is filled in at a lower part 1A of the riser 1, followed by supplying secondary air from a given position to permit a portion of the medium that has been fluidized to make it ascend through the riser 1 at a 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 via downcomer 8; and the collected fluidizing medium is then supplied with flowing air and returned to the riser 1. The cross-sectional area of the lower part 1A of the riser 1, in which the fluidizing medium S is filled in, is rendered to be larger than that of the remaining part of the riser 1.