A boiler apparatus is provided that generates a heat source capable of efficiently regenerating absorbing liquid while maintaining steam turbine efficiency to a maximum extent.The boiler apparatus includes: an economizer 54 preheating water serving as working fluid of a steam turbine system 500; an air preheater 69 installed downstream of the economizer 54; an evaporator 5 installed downstream of the economizer 54; an evaporator drum 6 using the water vapor from the evaporator 5 to generate saturated water vapor; a reboiler 20 using the saturated water vapor from the evaporator drum 6 to evaporate the absorbing liquid from a regeneration tower 23; a steam bypass pipe 15 connected to a steam pipe 14 through which the saturated water vapor from the evaporator drum 6 passes; a steam escape valve 16 installed in the steam bypass pipe 15; an exhaust gas bypass pipe 44 connected to an exhaust gas pipe 40 connecting the air preheater 69 with an absorption tower 26; and a butterfly valve 43 installed in the exhaust gas bypass pipe 44.