liquid being oxidized with oxygen contained in the gas and converted into calcium sulfate and said calcium sulfate precipitates to form the scale therefore an attempt has been made to lower the degree of excessive saturation of the absorbing liquid with gypsum as a countermeasure to the scale problem and it is a conventionally known method to add seeds of gypsum to the circulating absorbing liquid it is also known as means for increasing the effect of the gypsum seeds added to retain the gypsum seeds for a certain period of time in the absorbing liquid after said seeds have been added what is important to note here is that the present inventors have found through experiments that there are two types of scale namely a relatively hard scale consisting of gypsum as mentioned above and a relatively soft scale consisting of a mixture of calcium carbonate calcium sulfite and gypsum the formation of the former hard scale can be suppressed as by the addition of gypsum seeds described above but the precipitation of the latter scale is unavoidable and only encouraging the flow of circulating absorbing liquid is insufficient to eliminate the scale in the past an absorbing device has become inoperable after a few days of operation due to the soft scale deposited on the inner surface thereof and therefore it has been necessary when the absorbing device became inoperable to open the device remove the deposited scale and further wash the inner surface of the device with a liquid chemical the present inventors have found the following fact concerning the removal of such soft scale namely the present inventors have found that by operating the absorbing device while maintaining the ph value of an absorbing liquid.