In order to quantitatively evaluate actual circulation quantity of bed material extremely simply and to enhance accuracy of comparison of results of thermal balance examined through simulation or the like with actual operation results, time is measured which is required for bed material in a downcomer 5 to reach an upper predetermined height H1 from a lower reference height H0 during stopped feeding of fluidizing air to an external heat exchanger 7; a flow rate of the bed material as circulation quantity is determined from the time and an accumulated amount of the bed material based on an inner diameter D of the downcomer 5.