This invention relates to a waste gas treating apparatus, especially for treating a waste gas containing sulfur dioxide and carbon dioxide. It includes at least one washing tower. At least one fluid inlet is disposed on a lateral wall of the washing tower, so that a fluid is pumped into the washing tower by constant amount and drained out from a draining opening of a base of the washing tower. At least one gas inlet is disposed on the wall of the washing tower below the fluid inlet. The waste gas is injected into the washing tower so as to contact with the fluid and form bubbles which buoy upward to a top of the washing tower and are exhausted from a gas outlet thereof. The fluid serves to react with and absorb the pollutants contained in the waste gas. The products of the reaction are discharged out of the washing tower along with the fluid.