PURPOSE: To make cleaning work of a solution pipe easy to contrive uniform heating concentration of absorbing liquid in a flue by detachably mounting a baffle plate opposed to an upper space and a lower space at the upper and lower parts of a smoke box.

CONSTITUTION: A combustion gas allowed to flow in an upper space 21 and a lower space 22 of a flue 6 is collided with an upper baffle plate 27 and a lower baffle plate 28 to regulate the flow of the combustion gas, so that a large quantity of the combustion gas is prevented from being flowed and can be allowed to flow in the flue 6 uniformly. At the time of cleaning of each solution pipe 9a, 9g, the upper and lower baffle plates 27, 28 are removed to insert a cleaning nozzle 20 in the upper space 21 so at to clean each solution pipe with the use of a cleaning chemical sprayed from the cleaning nozzle 20. In addition, in the case where the upper baffle plate 27 and the lower baffle plate 28 are provided in the upper and lower parts of the outlet part of the flue respectively, the combustion gas allowed to flow in each space 21, 22 is regulated by each baffle plate 27, 28 to flow in the flue 6 nearly uniformly.