The degenerated limestone slurry from a scrubber, i. e., a perforated plate column without downcomer, was returned to a slurry holding tank where the loaded absorbent was regenerated before recycled to the column. The pH values of the slurry in the tank, the fractional removal of SO_2 and the coversion of limestone were experimentally determined as a function of process time, to obtain the condition under which the degenerated liquor is to be regenerated enough to maintain steady desulfurization with a closed-loop recycling of limestone slurry. As a result, it was found that the pH values of the liquor in the tank should be equal to or greather than those of 5.8 to 6.1 with fresh feeds of limestone into it to accomplish continual steady desulfurization. The optimal initial concentrations of limestone were found to be 3 to 5 wt%.