To check the worsening of fuel consumption as well as to secure an excellent nitrogen dioxide (NOx) abating effect by supplying a reducing agent, reducing an oxide of nitrogen in exhaust gases, to the upstream side of a catalyzer according to the actual flow rate of exhaust gas passing this catalyzer and the discharge of the oxide of nitrogen in the exhaust gas, in the case where an exhaust gas temperature is in an active temperature range of the catalyzer.

An as exhaust emission control system, it is provided with a NOx catalyzer 201 installed in an exhaust passage 102, a NOx sensor 104 installed at the upstream side of this catalyzer 201, a hydrocarbon supply injector 105 as reducing agent supplying means, and in this constitution, this HC supply injector 105 is driven by an electronic control unit 106. In brief, when an exhaust temperature is so judged as in an active temperature range of the catalyzer, NOx discharge rate is detected by the NOx sensor 104, and thereby a space velocity SV (exhaust gas flow rate passing through the unit capacity of the catalyzer) is calculated from those of engine speed and exhaust gas recirculation rate. Next, necessary HC/NOx ratio is found out of relations of this space velocity SV and HC/NOx ratio, and then necessary hydrocarbon is supplied from the HC supply injector 105.