PURPOSE: To prevent poor fluidization and make stable operation by replacing medium after the enlargement of the particle size of mediums in a fluidized bed has been judged from the steam temp., flow rate, and air quantity detected by a heat transfer tube placed in a fluidized bed.

CONSTITUTION: Layer temp. θ, the steam temp. θsi, and flow rate θs, at an inlet part, the steam temp. θso at an outlet part, and the air quantity G are stored in a heat transfer calculating part 21 in a pipe on a layer by means of a layer temp. indicator 17, a steam temp. indicator 15, a steam flow meter 16, a steam temp. indicator 18, and an airflow meter 19, and then average values in an hour and a heat transfer coefficient are calculated, and these calcu lated values are transmitted to a judging part 22 for abnormal particle size to control a control part 23 for medium supply and discharge. The part 22 starts to operate a constant volume feeder 12 and a belt conveyer 13 to change new mediums and also discharge old mediums by opening a medium discharge valve 10. After the mediums are charged or discharged for a fixed time, the part 21 is re-operated to monitor and control the equipment continually.