An air pollution control system for a coal combustion boiler includes: a NOx removing apparatus that reduces nitrogen oxide contained in flue gas emitted from this coal combustion boiler, an air pre-heater that recovers heat in the gas, a precipitator 15 that reduces particulates from the gas, a liquid-gas contact type desulfurizer that removes sulfur oxide by way of a limestone-gypsum method and reduces mercury oxide in the gas, a stack that releases the gas after SOx removal to the exterior, and a solid-liquid separator that extracts limestone-gypsum containing slurry from the desulfurizer to the exterior and performs a solid-liquid separation, where water separated in the solid-liquid separator is removed, and concentrated slurry is sent back to the bottom of the desulfurizer to make the gypsum concentration of slurry equal to or higher than 10 percent in the desulfurizer.