PURPOSE: To continuously quantify the soot contained in combustion exhaust gas, by guiding the combustion exhaust gas to an electric field and detecting the change in electric conductivity corresponding to the change in the concn. of the fine particles of the soot.

CONSTITUTION: When the soot fine particles in the combustion exhaust gas flowing in from a combustion exhaust gas inlet 5 collide with a cathode 3, said soot fine particles are charged. Electrons are diffused by the collision between the fine particles and the number of the charged particles increase and the soot fine particles charged at last collide with an anode 2. By this, a current flows and a weak current is detected and the change in electric conductivity corresponding to the change in the concn. of the soot fine particles is detected. By this method, the soot contained in the combustion exhaust gas can be continuously quantified.