To provide a boiler and its control method capable of surely drying even fuel including much moisture.

This boiler comprises a boiler body 2 for burning the fuel, an economizer 8 for recovering heat from an exhaust gas of the boiler body 2, an exhaust gas flow channel 4 for guiding the exhaust gas through the economizer 8, a bypass flow channel 4' for guiding the exhaust gas bypassing the economizer 8, an air preheater 5 for exchanging heat between the exhaust gas and the combustion air supplied to the boiler body 2, and a control section 15 for controlling a temperature of the combustion air at an outlet side of the air preheater 5 by adjusting flow rates of the exhaust gases guided by the exhaust gas flow channel 4 and the exhaust gas bypass flow channel 4'.