A gas introducing port 52 provided in a portion of a side wall 51b near a top (lid) 51a of a spray drying apparatus body, for introducing flue gas 18 to dry spray liquid 33a of dehydrated filtrate 33; a spray nozzle 54 hung down from the top (lid) 51a of the spray drying apparatus body, for spraying the dehydrated filtrate 33; a drying region L formed in the spray drying apparatus body, for drying the sprayed dehydrated filtrate 33 with the flue gas 18; a gas discharge port 53 provided in a portion of the side surface 51b near the bottom of the spray drying apparatus body, for discharging the flue gas 18 contributed to the spray drying toward a gas feed line, which is a main flue gas duct; and a wash nozzle 60, which is a washing unit for washing the spray nozzle 54 and the inside of the apparatus body with washing liquid are included.