To investigate an air pollution state due to an artificial exhaust gas by a radio active carbon density measurement in a plant even at a site where an air pollution substance measuring instrument cannot be installed.

A radio action carbon density in a plant is constant anywhere on the earth unless a special reason has been given. However, a gas exhaust by combustion of a fossil fuel does not include ratio active carbon; therefore, the radio active carbon density in the plant grown in an area containing much exhaust gas of the fossil fuel decreases with an increase in a quantity of exhaust gas. Hence, by measuring the radio active carbon density in the plant, the air pollution degree due to the an artificial exhaust gas in that bleeding area can be determined.