To provide a circulating fluidized bed combustion apparatus in which variations of a combustion state of circulated particles due to external turbulence in an in-riser fluidized bed are suppressed, and maintenance control of a stable combustion state is ensured, and further exhaustion of harmful gas and unburnt gas is suppressed.

A circulating fluidized bed 11a is of a circuit structure for circulating particles, in which it comprises a downcomer 14 composed of a riser 12a, a solid/gas separator 13a, and a return pipe 15a. Two means are adopted, in which the other end of the foregoing return pipe 15a is installed on a riser wall having a height above a concentrated layer region A in the riser 12a to eliminate the influence of turbulence of circulated particles with respect to the concentrated layer region A, so that variations of a combustion state are adjusted with the aid of control factors of the amount of supply of wet substances and primary air, etc., and hence maintenance control of a stable combustion state in the riser is ensured.