PURPOSE: To decrease pollutant substance by decreasing the concentration of all noxious sulfurous oxides especially discharged when heavy oil of low quality is burned, by burning heavy oil by uniformly mixing approximately the same quantity of calcium as sulfur content contained in heavy oil, with the heavy oil.

CONSTITUTION: At the time of combustion, first, approximately the same quantity of calcium as sulfur content contained in heavy oil is uniformly mixed with heavy oil by stirring them. The mixed heavy oil is reserved in a tank 1 of a predetermined capacity, continuously being mixed, and the necessary rate of oil is injected to be burned in a combustion furnace 2. At this time, the air is fed from a blower 17 controlling the air rate to the minimum that is necessary for combustion. By this method, the sulfur content in the heavy oil forms gypsum by chemically reacting to calcium, as a result, sulfurous oxides in exhaust gas is removed. The above-mentioned mixing of heavy oil with calcium is taken place by stirring in the tank 1 in such a manner that a sticking mixer 4 are provided in the tank 1, and their tops and the bottom of a tank 1 is connected by a circulating pipe 5, while a pump 6 is provided in the middle part of the circulating pipe 5.