Provided are drain tank storing predetermined amount of drain from cooler; alkaline aqueous solution supply unit with aqueous solution adjustment tank receiving and storing part of drain in drain tank in predetermined amount, solid alkaline agent supplier supplying solid alkaline agent to produce alkaline aqueous solution, alkaline concentration controller controlling solid alkaline agent supplier to regulate alkaline concentration of alkaline aqueous solution, and pump supplying alkaline aqueous solution to exhaust gas entry side of cooler; and alkaline supply control unit with impurity sensor downstream of succeeding cooler, drain pH sensor obtaining detected pH of drain in drain tank, and supply controller controlling alkaline supply amount supplied to exhaust gas entry side of cooler based on detected impurity value to make detected pH to set value.