This fluid bed reactor prevents catalyst accumulation in a recessed portion of the inner wall of the reactor and prevents device deterioration and adverse effects on reactions. Further, the fluid bed reactor prevents rapid generation of heat due to outside air contacting a catalyst accumulated in the recessed portion of the inner wall of the reactor. Also, the fluid bed reactor prevents catalyst particle accumulation on a protruding portion inside of the reactor, and prevents deterioration of material quality of the device and adverse effects on reactions. Furthermore, the fluid bed reactor prevents rapid heat generation due to outside air contacting catalyst particles accumulated on the protruding portion of the reactor.