PURPOSE: To continuously detect the quantity of smoke which is being exhausted with high accuracy, by electrifying soot particles in exhaust gas by using an electric force field generating means, and detecting the smoke quantity, based on its electrifying current.

CONSTITUTION: In the vicinity of an exhaust valve 8 of a exhaust passage 7 of an engine 1, an electric force field generating means 17 for generating an electric field or a magnetic field is provided, and also, in the electric field or the magnetic field generated by this electric force field generating means 17, a pair of electrodes 20, 20' are provided. When exhaust gas passes through the inside of said exhaust passage 7, Coulomb force and Lorentz's force are generated in soot particles in the exhaust gas, the soot particle and an electron move to a cathode side electrode 20, and an anode side electrode 20', respectively, and a current flows. This current is detected by a current value detecting means. Also, based on this detected current value, the quantity of smoke which is being exhausted is detected.