A method of controlling a system having an internal combustion engine, a filter in an exhaust passage, a reduction catalyst downstream of said filter capable of reducing nitrate oxide contained in the exhaust gas with reductant supplied thereto is provided. The method may include, in a first mode, combusting a first amount of particulate matter trapped in said filter and supplying a first amount of reductant to said reduction catalyst when an amount of particulate matter trapped in said filter is less than a predetermined trapping amount (α). The method may further include, in a second mode, combusting a second amount of particulate matter that is greater than said first amount of particulate matter and supplying a second amount of reductant to said reduction catalyst when an amount of particulate matter trapped in said filter is equal to or greater than said predetermined trapping amount (α).