Investigation of SO₂ absorption was performed with unreacted absorbent on bag filter which is usually installed in downstream of semi-dry absorption process for flue gas desulfurization (FGD) to remove SO₂ in emission gas of incineration. In this study, Ca(OH)₂ was used as an absorbent as well as NaOH. SO₂ absorption was examined with varying temperature, flowrate and concentration of SO₂. NaOH addition to Ca(OH)₂ and effects of amount and concentration of Ca(OH)₂ were also examined, Although SO₂ temperature did not affect starting detection time of unabsorbed SO₂ on Ca(OH)₂, higher SO₂ temperature increased absorbed SO₂ amount. However, increase of SO₂ flowrate initially decreased starting detection time of unabsorbed SO₂ on Ca(OH)₂, but insignificantly influenced absorbed SO₂ amount. Similarly to SO₂ flowrate, SO₂ concentration also initially decreased the starting detection time. On the other hand, more amount of Ca(OH)₂ increased the unabsorbed SO₂ detection time and total absorbed SO₂ amount. Higher Ca(OH)₂ concentration caused higher absorbed SO₂ amount. In especial, absorbed SO₂ amount was soared at the use of 30 wt% Ca(OH)₂. The addition of NaOH to Ca(OH)₂ could lower absorbed SO₂ amount at the corresponding concentration of Ca(OH)₂. Thus, this study would contribute to understanding secondary SO₂ removal in semi-dry SO₂ absorption, and support effective selection of absorbent type and use of proper amount of absorbent under the field operation conditions